What makes the clutches slip are the "energy conserving" additives put in many car oils. But ..... and this is a very big "But"..... those additives are only put in the lighter car oils, generally 30wt. and lighter. They don't bother putting them in the 40 and 50 wts. I'm guessing they don't help an oil that thick be any more "energy efficient". I've used car oils (thick ones) in all my bikes for decades and never had a wear or clutch slippage problem. I can think of far better things to waste my money on than fancy oil, lol.