Let's get the record straight. There is nothing wrong with M1 15w50 oil or any other "car oil," even if it isn't labelled as motorcycle-specific. There have been numerous credible tests comparing this with M1 VT 20w50 and there is little difference in the formulas. Ref: Motocycle Consumer News, American Iron, and Quickthrottle. M1 15w50 was tested for viscosity retention and many other criteria and given a thumbs-up in all three tests. There is nothing unique about a motorcycle engine, whether air- or water-cooled, that would dictate a radical change in oil formulation. Oil temperatures may run hotter in an air-cooled engine without an oil cooler, but any modern synthetic oil can handle these temperature extremes. BTW, I've had discussions with Mobil 1 engineers and they verify what I'm saying.

I've known many people who've used this oil long-term with no problems. I have two friends with functionally identical 2000 TC88's, one an FLHTC with 60k miles using M1 15w50 and the other an FLHT with 50k miles using Amsoil 20w50 MC oil. These were upgraded to 95ci at the same time, and upon inspection of the old jugs and pistons there was no indiction of wear from either bike, either visual or after micrometer measurements were made. Even the piston coating was still intact.

Another case was my old '96 RK (Evo) that logged 106k miles on Royal Purple 20w50 "car oil." I performed compression tests on that bike yearly, and in 1998 static compression was 155F/160R. In 2006 it was 160/160, and the heads have never been removed from the bike. It is still running with >115k at last check. If "car oil" is destructive, I'd like to know when will it destroy this bike?

 

Thanks ICLICK, that makes me feel a bit better about running Mobil 1 15w50.