lta01 wrote:I'm not an oil expert, and don't profess to be. what I can tell you is this :
Over the last 15 years, I've restored and run 6 Gs1000's, a CBX, Z1300 , both Benelli sixes, Z1a, Z1r , Guzzi Lemans 3 and a few others I cant remember.
I tried Mobil 1 fully synth in nearly all of them ( bought a drum of it ). over time, I experienced weeping gasket joints and starter clutch slippage on more than half of the bikes. Ditched the fully synth, went back to semi, problems gone.
Don't ask me to explain why, I'm just passing on my real experience across a wide range of bikes , engines, cranks/bearings, etc.
I think the most telling was the starter clutch slippage, undisputable and like a light switch gone when reverting to semi synth.
I've read on many forums that fully synth is "too slippery" for older roller bearing applications. A person I do regard as a real bike expert once suggested the starter clutch issue was to do with drainage, modern engines don't pool oil in the sprag clutch area the same as older designs.
There you go.... will watch with interest.
I think the starter clutch issue is the same one as roller bearings sliding, the synthetic oils have a stronger film and don't allow the starter clutch to break it down and grip. On the roller bearing issue, I feel if the oil film allows the bearings to slide, there won't be any wear, because of the oil film, if the film breaks down they would just roll again. I've built S and S Harley engines, (roller bearings, no gearbox, no clutch) and S and S themselves told me that 5w50 Mobil 1 car oil was fine in their engine - straight from the horses mouth in their technical department in the land of litigation.
Car synthetic oils are designed to run a car engine, ie shell bearings, no clutch, no gearbox, I wouldn't use them in a Jap bike engine for that reason. As stated above, there is a rating for oils designed to work with a wet clutch, the scientists that formulate this stuff are lot cleverer than me and if they say their oil is, or is not suitable to use with a clutch, I am going to believe them. Likewise for gearboxes, ie "motorcycle oil" because a gearbox gives oil a different life than the bearings of the engine.
9. 4-stroke motorcycles usually need special oils
Many people think that a 4-stroke motorcycle requires a regular motor oil like a car. Both have 4-stroke engines after all. In reality, however, in most modern motorcycles the motor oil is also responsible for lubricating the transmission and the wet clutch (my note - not a Harley). Such motorcycles need special motor oils that can perform these duties as well. Modern passenger car motor oils contain way too much friction modifier additive to be able to handle these tasks. The solution is the appropriate 4T oil that meets the motorcycle manufacturer’s specifications. In most cases this means a JASO MA type oil.
10. Motor oils usually aren’t too thin when hot but they are too thick when cold
A lot of people like to choose high viscosity grade motor oils assuming they provide better protection, especially when hot. They worry that a hot, low viscosity grade motor oil would not create a strong enough oil layer and engine wear would be more significant. The truth is, however, that most of the wear occurs at cold starts. An oil with a lower viscosity grade can provide better protection at cold start because it takes less time for it to reach its optimal viscosity (in layman’s terms thickness) so the engine runs less time not fully protected. To also address the problem of the thinner oil film at high temperatures: the oil film’s strength is not determined solely by its thickness. Modern synthetic oils provide very thin but very strong oil films. If that was not the case there could not be 0W20 and 5W20 oils at all, since they would be incapable to protect the engine with their film thickness alone.
So if you use a car fully synthetic, you clutch may well slip, the thinner film, may make you engine appear to rattle, or find it's way out of gaskets, and your starter clutch may slip (because of the high strength film).