Comprehensive look at MOGAS vs 100LL for Rotax iS engines

[stathis]

Hi everyone,

This is a subject most of you are already familiar with, but I wanted to take a deeper dive and pull together everything that I could find from Rotax documentation, owner forums, and articles into one place. The result is a paper (and a summarized blog post) that looks at both the technical and practical sides of using MOGAS and 100LL in the Rotax iS-series engines.

It covers:

• Rotax’s official guidance from Service Instruction (SI) on Selection of suitable operating fluids for ROTAX® Engine Type 916 i (Series), 915 i (Series), 912 i (Series), 912 and 914 (Series)
• Maintenance differences (oil change intervals, plug life, gearbox service)
• How Decalin helps mitigate lead buildup when running avgas
• Ethanol and vapor-lock considerations when using auto fuel
• The pros and cons of mixing small amounts of 100LL with MOGAS to reduce vapor pressure
• Future unleaded options like Swift UL94, G100UL, and Swift 100R, and the FAA’s 2030 phase-out goal for leaded avgas

For example one interesting insight is that the Sling Pilot Academy itself routinely uses a blend of 100LL and MOGAS in their Rotax-powered aircraft, particularly in hot weather or when ethanol-free auto fuel isn’t readily available. That small percentage of avgas (typically 30%) lowers vapor pressure and helps prevent vapor lock while keeping most of the benefits of running unleaded fuel — a practice echoed by many experienced Rotax operators.

If you’re flying a 912iS, 915iS, or 916iS, you might find this a useful reference or at least a good conversation starter.

You can read the post and download the paper here.

Stathis

[lutorm]

There's a typo in your link, it's missing the colon after https.

Looking at the latest SI for fuel I see it saying "Min RON 98" for the 916 (the other engines have min RON 95 as you state in your post.) I was interested in whether avgas ASTM D910 91/96UL as available in Europe is acceptable. According to the spec sheet it has RON 98.9 but the only avgas listed in the Rotax SI is 100LL. Curious.

[stathis]

Thank you, the link is fixed.

Regarding your question my understanding is that although Rotax lists 100LL explicitly in the SI tables, 91/96 UL is chemically and operationally equivalent to unleaded avgas meeting or exceeding the RON 98 threshold. Its spec sheet RON ≈ 98.9 meets the minimum requirement. The omission in the SI’s “Approved Fuel” list is mainly because Rotax seems to treats “AVGAS 100LL” as the generic avgas reference fuel under ASTM D910, while 91/96 UL is a regional unleaded variant.

[lutorm]

Yeah I guess you're right, it's just weird that they note all the various local Mogas standards, like "Ukraine" for 916, but not local avgas variants. Maybe Ukraine is a large market for 916-driven UAVs right now so they wanted to get that in, but I'd think they'd be familiar with the avgas variants available in Europe, too.