the subject of oil preference is one of the most hashed out subjects ever, so what the hell its sunday morning, its cold out, the only oil you should not put in a oil bath clutch bike is anything with Teflon or any other type of slick additives, other than that I'm not sure how the bikes eng could ever tell the difference between motorcycle specific oil and normal car oils, now amsoil, motul, and I'm sure 10 or 12 other (motorcycle) oils are great oils, but you generally do have to buy these oils at a bike or speed shop, and pay dearly for them, and I'm not going to argue weather they are worth the money, in the many years of bikes I have had, I have used about everything at one time or another, 5-30 10-30 10-40 15-40 20-50, in both synthetic and normal (petro) based. at times it would seem when the oil gets a lot of time oil miles on it the clutch may react a little different, no slip, but maybe a little grabby, but it was hardly noticeable, and the type or weight of the oil never seemed to make any difference, now personally if I can get a coupon and Walmart has it I will use 15-50 Mobil 1, and if not a good name brand diesel 15-40, Rotella is a good oil, in ether synthetic or normal, but if available I try to go with Valvoline, or Havoline, or Castrol 15-40 diesel, reason being although the Rotella is a good oil it seems to get shit up (gets dirty looking) faster than the others, I also use the same oil in the Harley's eng, ........now if you have a bike that has had the dogshit ridden out of it and the motor smokes or make unusual noises, or leaks like a sive, all bets are off, but the heavyest crap you can find in it 50 or 60 weight