TL;DR: A season of real cross-country hours changed my plan, not just my thinking. I’m dropping the effort to build a home MOGAS setup and going 100LL as my default until low-lead or unleaded avgas is actually available where I fly — my own logbook already exceeds the 30% leaded-fuel threshold on cross-country trips, so the hybrid approach from October never had a chance to pay off. A firsthand safety briefing on vapor lock, a harder look at MOGAS logistics, and new data on the 916 iS itself — including confirmation this engine has no oxygen sensor, that XPS full-synthetic oil is approved for leaded fuel, and that its gearbox interval doesn’t actually shorten on 100LL — all pushed me here, and brought Decalin back into the plan along the way.
Back in October, I wrote A Pilot-Owner’s Guide: MOGAS vs 100LL in Rotax iS Series Engines, and it’s become one of the most-read posts on this blog. The plan I landed on then was a hybrid: run 100LL with Decalin while getting the airplane sorted, then build out a home MOGAS fueling setup and use MOGAS as my day-to-day default, saving 100LL for cross-country trips where MOGAS wasn’t available. That was where the analysis pointed at the time.
Since then, I’ve accumulated more of what actually moves these decisions forward — hours, data, direct experience — along with source material and conversations that weren’t part of the original picture. Some of it confirms what I wrote in October. Some of it shifts the conclusion. And one detail about my own engine, I had flatly wrong.
This is where that thinking stands now. I’ll walk through what changed my view, set the record straight where I need to, and lay out my current position now that a few more hours have given the original analysis something to run against.
Where I’ve landed: I’m not building the home MOGAS setup. I’m going 100LL as my default until low-lead or unleaded avgas is actually available at the airports I fly to, Decalin is part of the routine, and the 2030 transition is the real endpoint I’m planning around, not a distant possibility. Here’s how I got there.
What Changed My Mind
A safety briefing I sat in on. I attended a session that covered two serious vapor lock incidents involving winter-blend MOGAS, even in aircraft running a blend with some 100LL mixed in. I can’t get into specifics — the briefing wasn’t public, and the details aren’t mine to share. What I can point to is the outcome: the operation’s own response was to move back to straight AVGAS, and stay there until they can secure SWIFT 94 through the airport authority. When an operator running a fleet of aircraft on the line makes that call, it’s worth taking seriously — that’s not a hypothetical risk, it’s a documented operational decision by people with real exposure to it.
The logistics are worse than I gave them credit for. My October post treated MOGAS as a straightforward “cleaner fuel, use it when you can” proposition. What I underweighted is how much friction sits underneath that sentence in practice. Ethanol content is disclosed clearly enough at the pump — that’s not the problem. The actual problem is availability: genuinely ethanol-free MOGAS is hard to find outside specific stations, and most owners end up with whatever blend is locally available rather than a fuel they’ve chosen deliberately. Add to that the seasonal RVP-blend changes — the MOGAS you buy in July isn’t chemically the same as what you buy in January, which is exactly the mechanism behind the vapor lock incidents from the briefing — plus real storage and shelf-life constraints for anyone who doesn’t burn through a tank quickly. None of this is exotic; it shows up at the airport-operations level too. ACRP Research Report 284, a 2026 airport-transition guide sponsored by the FAA, documents what U.S. airports have actually experienced trying to add an unleaded option alongside 100LL: price premiums of $1–4 per gallon, tank-conversion delays, inconsistent supply logistics, and demand that in several case studies declined over time as pilots reverted to whatever fuel was simply easier to get. If airports with dedicated staff and federal funding find this to be a genuine operational headache, it’s reasonable to expect the same friction for an individual owner sourcing MOGAS trip by trip.
What I’ve come to understand about XPS. Rotax’s own documentation confirms XPS Full Synthetic 5W-50 was formulated for Rotax-powered aircraft using either leaded or unleaded fuel — the result of two years of development, forty tested fluid formulations, over 20,000 gallons of aviation fuel burned in testing, and more than a dozen fired engine test runs. That’s worth noting against what I wrote in October, where I said semi-synthetic oil was the requirement for any avgas use — XPS is Rotax’s own current answer for leaded-fuel operation, and it’s a full synthetic, so that guidance has clearly moved on since then. Conversations within the Rotax community add a useful detail on what the testing actually validated: it wasn’t just general compatibility, but specifically confirming the oil holds up under 100LL without gumming or sticking components like piston rings — a real failure mode with older oil formulations under sustained leaded-fuel operation. I’ll say plainly that I don’t have independent data on exactly how XPS handles lead in suspension versus older semi-synthetic formulations — that’s a detail Rotax mechanics themselves still debate — but the ring-sticking and gumming problem is the one I was most worried about, and that appears to be solidly addressed.
The math that actually settles it. This is the part that changed my conclusion the most, and it’s not abstract — it’s my own logbook. Rotax’s oil-change interval doesn’t shorten because you choose 100LL; it shortens once leaded fuel exceeds 30% of total operating time, full stop, regardless of intent. Looking at my last 50 hours, nearly all of it cross-country flying, fuel availability was dictated by whatever was on the ramp at each stop — not by what I would have preferred. I was well over that 30% threshold despite actively trying to use MOGAS when I could. For a mission profile built around cross-country flying, chasing MOGAS availability doesn’t buy back the 50-hour oil interval. I’m locked into it either way. That’s the moment the cost-benefit of fighting MOGAS logistics stopped making sense for how I actually fly.
Correcting the Record: Oil and Decalin
October’s post recommended Decalin as a standard part of running 100LL, following what’s common practice in the Rotax owner community. Having looked at this more carefully — including reading through a good stretch of owner forums and product reviews — I want to walk through what the evidence actually shows, and why I’ve ended up reversing my position rather than just softening it.
What the evidence on Decalin actually shows: a lot of satisfaction, no real data. Owner reviews and forum accounts are consistently, almost uniformly positive — cleaner plugs, less fouling, smoother running, people who’ve used it for years and won’t go back. That pattern is worth taking seriously. But it’s satisfaction reporting, not measurement. Rotax itself has never tested it — the company acknowledges Decalin exists and that owners use it, without putting any data behind the claims. The one quantitative number I found — 1,098 ppm of lead in one owner’s oil after 91 hours, all of it treated with Decalin at the label rate — doesn’t actually resolve anything, because there’s no baseline for what that same engine would have shown over the same 91 hours without Decalin. It shows lead was still accumulating; it doesn’t show how much less accumulated because of the additive. So the Vulcan summary is: broad anecdotal satisfaction, and zero independent or controlled data.
Given that — and given the oxygen sensor correction below — I’m going back to using it. Normally “no data” would be a reason for caution. But the calculus here is different: the downside risk I was most worried about with Decalin was the oxygen sensor maintenance burden, and that risk doesn’t exist on this engine, or on any of Rotax’s factory iS-series engines — there’s no O2 sensor anywhere to foul or clean. With that concern removed, I’m left with an additive that has a large, consistent body of anecdotal support, a low cost, no credible reports of harm, and a plausible mechanism — converting lead to a softer phosphate compound — that Rotax itself acknowledges without disputing. That’s not the same as proof it works, but it’s a low-risk bet with a lot of satisfied users behind it, and the one reason I’d talked myself out of it no longer applies.
Here’s what I got wrong. In October, I wrote that Decalin’s label instructs owners to clean the oxygen sensor periodically to remove lead deposits, and I said “if your Rotax has an oxygen sensor — as all iS engines do.” That second part isn’t true. I went through the complete sensor list in the current Line Maintenance Manual for the 916 iS — ambient air pressure and temperature, cam and crank position, cooling temperature, knock, manifold pressure and temperature, oil pressure and temperature, and throttle position — and there is no oxygen or lambda sensor anywhere on this engine. I widened the search to confirm this holds across the whole factory iS family, not just my engine: the EASA Type Certificate Data Sheet for the 912/915/916 iS series and multiple owner discussions of the Garmin CAN bus interface confirm the same sensor architecture across all three. One nuance worth flagging — a couple of aftermarket fuel-injection conversion kits for the older *carbureted* 912 do use an oxygen sensor as part of their own, non-Rotax injection design. That’s a different product built by different companies, not Rotax’s factory iS system, and it’s easy to conflate the two if you’re not careful about which engine and which injection system is actually being discussed. The 916 iS’s FADEC controls mixture through manifold pressure, manifold temperature, RPM, and knock feedback — a model-based approach, not one that reads exhaust gas composition in real time. Rotax engines don’t run catalytic converters, so they have no reason to hold a stoichiometric mixture, which is the only reason road-going engines need an O2 sensor in the first place.
Practically, this means the entire “clean your O2 sensor every 20 hours” consideration — which I’d folded into the Decalin discussion as a maintenance cost — doesn’t apply to this engine at all. It’s one less thing to worry about, not one more, and I was wrong to suggest otherwise.
Design Changes and Long-Term Lead Exposure
I went looking for a proper long-term study on what sustained 100LL exposure actually does to a Rotax engine over its life, expecting to find one. I didn’t. What exists instead is Rotax’s own maintenance-interval guidance, which functions as the closest thing to an engineering answer: shortened oil-change intervals once leaded fuel crosses 30% of operating time, and — on older engine generations — a shortened gearbox inspection interval as well. That’s not nothing; it’s Rotax telling owners, through the maintenance schedule itself, where the real wear risk sits. But it’s guidance built from field experience and engineering judgment, not a published long-term study, and I think it’s more honest to describe it that way than to imply there’s a dataset behind it that doesn’t exist.
Here’s something I didn’t expect to find when I went looking: my engine may already have some of this designed out. The gearbox and overload-clutch failure mode that shows up in older-generation Rotax discussions — lead settling and packing into the overload clutch assembly over hundreds of hours, tied to a specific clutch part that lacked any way to drain it out — was addressed at some point in Rotax’s engine evolution by adding drain holes to let lead escape rather than accumulate. I checked the current Line Maintenance Manual for my engine specifically, and the gearbox and overload clutch inspections are scheduled at a flat 1,000-hour interval, with no shortened interval tied to fuel type anywhere in that table. That’s meaningfully different from what I wrote in October — where I cited a 600-hour interval on 100LL versus 1,000 on unleaded — and from the split intervals described in older Rotax service guidance for earlier engine generations more broadly. It lines up with the idea that the redesigned clutch is standard equipment on my engine. I want to be careful here — the MML doesn’t spell out “we fixed this because of lead,” it just shows the interval, and I’m inferring the reason from a design change described elsewhere. But it’s a specific, checkable detail, not a guess, and it’s a genuinely encouraging one: the failure mode that worried me most on the mechanical side appears to already be addressed on this engine, not something I need to actively manage.
Beyond that, what’s out there is mechanic and owner field experience — useful as color, not as authority. Rotax instructors and shops describe valve sticking and gearbox lead accumulation as real but manageable risks when owners actually follow the shortened oil interval, and describe the failure mode as showing up mainly in engines where that interval wasn’t respected. I don’t have a way to verify how common that is, so I’m treating it as a directional data point rather than a citable fact.
Where this leaves me: the long-term lead question doesn’t have a clean answer, but the practical version of it — will this specific engine’s gearbox and oil system tolerate the leaded-fuel operation I’m actually doing — looks better than it did in October, on the strength of a design change that predates my ownership and a maintenance interval I’m already following.
Ethanol and Fuel Hoses
One thing I didn’t examine closely enough in October: what ethanol actually does to the rubber components in a fuel system, and whether that’s a live concern for this airplane specifically.
The common shorthand — “ethanol degrades rubber fuel lines” — turns out to be an oversimplification. Compatibility depends heavily on which specific elastomer compound is actually in the hose, not on “rubber” as a category. Testing on different compounds shows a real spread: some nitrile and HNBR-based materials hold up reasonably well to ethanol-blended fuel, while others — neoprene among them — show pitting, leaching, and shrinkage under the same exposure. Separate testing has found some HNBR formulations swelling well past acceptable limits, while fluorocarbon and fluorosilicone compounds perform best. The practical upshot is that the question isn’t “does my aircraft use rubber fuel hose,” it’s “which rubber.”
That distinction shows up directly in the automotive world these two hose specs came from. SAE J30R7 is generally considered a low-pressure, legacy fuel-hose spec — suitable for older carbureted systems, but multiple sources are explicit that it isn’t rated for ethanol-blended fuel and shouldn’t be used with it. SAE J30R9, by contrast, uses a fluoroelastomer lining specifically engineered for ethanol-extended gasoline, with far better permeation resistance. Same general hose category, meaningfully different ethanol tolerance.
Here’s where my own airplane comes in. Rather than guess, I pulled the actual fuel-system diagram from my POH. Every flexible hose run in the system — both 3/8″ sections and the 5/16″ run — is specified as SAE J30R9. There’s no J30R7 anywhere in the fuel system as built. That means the ethanol-degradation risk that’s a legitimate concern for builders running generic or automotive-grade low-spec hose simply isn’t a live issue for this airplane’s fuel lines — the system was speced with the ethanol-rated grade from the start.
None of this changes the vapor lock or seasonal-blend concerns from earlier — ethanol tolerance in the hoses and volatility behavior of the fuel itself are two different issues, and MOGAS’s logistics problems don’t disappear just because the hardware handles it fine. But it does mean I can cross “my fuel lines might not tolerate ethanol” off the list of reasons to avoid MOGAS. That one, at least, was never actually a risk for this aircraft.
The 2030 Transition Changes the Math
None of the above matters as much if the leaded-fuel exposure window is only a few years long rather than the life of the engine — so it’s worth being precise about where that timeline actually stands, rather than treating “2030” as a vague, maybe-it-happens-maybe-it-doesn’t date.
I understand the skepticism. There’s a long history in general aviation of unleaded-fuel deadlines sliding, and plenty of pilots have watched enough of those slip to be reasonably unconvinced this one is different. Ben Visser, a former Chevron fuels engineer, writing in General Aviation News, addresses that skepticism directly and makes a case for why this deadline is harder to push than past ones — not because of regulatory willpower, but because of supply. Innospec, the last Western manufacturer of tetraethyl lead, has announced plans to stop producing it in the 2028–2029 timeframe, with existing inventory expected to support blending only through the 2030 sunset. That’s a physical constraint, not a policy preference — nobody can extend a deadline past the point where the additive simply isn’t being made anymore.
The regulatory side backs this up. The FAA released a draft Transition Plan to Unleaded Aviation Gasoline in January 2026 targeting the end of 100LL in the lower 48 by the end of 2030, and unleaded fuels are already rolling out incrementally rather than sitting on paper — GAMI’s G100UL is being sold at a small number of airports, Swift’s 100R is expanding, and a third fuel from LyondellBasell/VP Racing is working through FAA testing with fleet authorization anticipated for 2027. Grant assurance rules also mean 100LL access at my home airport won’t disappear early — airports can’t drop it before the deadline unless a certified replacement is already widely available there — so the risk isn’t losing 100LL prematurely, it’s losing it on schedule.
For my mission profile, that turns into a real number rather than an abstraction. At roughly 200 hours a year, four years to 2030 is about 800 hours of leaded-fuel exposure worst case — and likely less, if an unleaded option becomes available at my airport before the hard deadline. Against a 2,000-hour TBO, that’s under half the engine’s life spent on leaded fuel, even in the worst case. Combined with the design changes and maintenance intervals covered earlier, the actual exposure window looks narrower than it did when I was thinking about this as an open-ended commitment.
I’d be overstating it to call the transition itself frictionless, though. ACRP Research Report 284, a 2026 airport-transition guide sponsored by the FAA, documents what’s actually happened at airports further along in this process than mine: price premiums, tank-conversion costs, inconsistent unleaded fuel supply, and demand that in some cases quietly declined as pilots reverted to whatever was easier to get. One case study specifically noted flight schools hesitant to fully commit to unleaded fuel because of an ongoing valve-seat-recession investigation involving UL94 — a documented, industry-wide version of exactly the kind of caution I heard firsthand at the safety briefing earlier. The 2030 deadline being real doesn’t mean the path there is smooth; it means the endpoint is fixed, even if the road to it stays bumpy.
Net effect on my thinking: the leaded-fuel exposure I’m accepting by staying on 100LL isn’t open-ended. It has a real, foreseeable ceiling, and that ceiling is closer than the length of this engine’s service life.
Where I Land Now
Here’s my current position, and the reasoning that gets me there:
I’m sticking with 100LL, period — not just for cross-country flying, and not primarily for reasons of cost or convenience. Safety is the real driver: the vapor lock incidents from the safety briefing weren’t a mission-profile problem, they were a fuel problem, and that risk doesn’t go away just because I’m flying locally instead of cross-country. My mission profile reinforces the decision rather than being the basis for it — I don’t avoid the 30% leaded-fuel threshold regardless of intent, so chasing MOGAS availability trip to trip doesn’t buy back the 50-hour oil interval, and the logistics cost of hunting down ethanol-free MOGAS, tracking seasonal blends, and managing storage doesn’t pay for itself.
I’m going to start using Decalin. There’s no controlled data behind it — just a large body of consistent owner satisfaction and a plausible mechanism Rotax itself acknowledges without disputing. Normally I’d treat that as a reason for caution, not adoption. But the one concrete downside I’d built my earlier hesitation around — the oxygen sensor maintenance burden — doesn’t exist on this engine, or on any factory Rotax iS engine. With that risk removed, a low-cost additive with a decade of satisfied users behind it is a reasonable bet, even without a study to back it.
The gearbox and clutch design already on this engine, plus the oil interval I’m already following, cover the mechanical risk that used to worry me most. That’s the part of this whole question that turned out to be in better shape than I gave it credit for in October — not because I changed anything, but because the engine and the maintenance schedule were already built around it.
The 2030 deadline is real enough to plan around, not just note in passing. Roughly 800 hours of leaded-fuel exposure worst case, against a 2,000-hour TBO, is a bounded cost I’m comfortable accepting rather than an open-ended one I need to engineer my way out of.
None of this is final. This is where the analysis and the last several hundred hours have actually taken me — not where I expect to stay forever. If you’ve run a different mission profile, a different engine generation, or you’ve had a different experience with Decalin, XPS, or MOGAS logistics, I’d genuinely like to hear it. This is exactly the kind of thing that gets better with more data points than my own.
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