Aero Shell 4 Plus

Yes, Rotax early-on recommended Mobil 1 Racing 4T Motorcycle 10W-40 Full Synthetic.  Rotax now only recommends Aero Shell 4 Plus, but I think this is strictly a financial agreement with Shell and has no technical basis.  I use Mobil 4T in my 912ULS RV-12 since new and now have 800TT.  Compression is like new and runs like a champ.  Synthetic oil is much better for elevated temps.  Last time I bought Mobil 4T it was $10 bottle.  I use 93E10 Mogas and change oil at 75 hours...

You are correct that oil must have additives that work with high shear pressures associated with transmission gears.  Mobil 4T is correct for this application where oil is shared between engine crankcase and gearbox. 

Hi James,

FYI: Here in Australia, most suppliers are charging more for Mobile 1 4T  than what you can get  Aeroshell Oil Sport Plus 4 for. I did find one advert for Mobil, very slightly cheaper than the Aeroshell. (brought all pricing to a per litre cost to account for differing volumes). So little or no advantage is going for what is no longer recommended by Rotax.

Rotax has their own oil specification, RON 424, and Shell is the only manufacturer that has made the investment in formulating and testing an oil to meet that standard. The oil must perform well for the engine, gearbox, and have a friction coefficient suitable for the overload clutch. In addition, the oil is formulated to hold lead in suspension to keep the engine clean when leaded fuel is used. Every business relationship has a financial component, but this is a relatively small market, so to get a major player like Shell on board it would almost have to be an exclusive opportunity.  Its my opinion that the advantages of using Aeroshell 4 Plus are real. 

https://aviationoiloutlet.com/aeroshell-sport-plus-4/
Cheaper and free shipping. But still $60 more for the case than I paid last November. 

Mobile One Racing 4T is a motorcycle oil which has ZDDP additive plus others. The ZDDP helps coat and protect contacting metals. Mobile One is a full synthetic which is okay for unleaded auto fuel, but not for 100LL. A full synthetic will not suspend the lead in 100LL and you pay a big price later on as the lead spreads. Our oil is designed the way it is like some motorcycles because the engine oil is used in the gearbox at the same time. If you can get it use the Aero Shell. If not a good rated Motorcycle oil like the Mobile One 4T or even the Amsoil motorcycle full synthetic oil will work, but not with 100LL.