Clutch slippage is not the only consideration when concidering auto v motorcycle oil. Back in the day, 1980's, auto oil had plenty of zinc. It has been found zinc damages catylistic converters, so the zinc content has been reduced to half the old level. Motorcycle oil has a high zinc content to help lubricate older style cam and tappets used on many motorcycles. Note the zinc content on Moble1 motorcycle oils.
^ From a ~9 year old thread, here's what you're looking for on the back of oil containers. If you have trouble finding regular engine oil (not motorcycle specific) that's non-energy-conserving, I've found that most brands that carry a full synthetic for European vehicles fit the bill.