Comprehensive look at MOGAS vs 100LL for Rotax iS engines

stathis · Post by **stathis** » Mon Nov 03, 2025 7:16 pm

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