This can be risky. Even though you can and will find car oil with the same viscosity ratings (i.e. 10w40) as for motorcycle engines, the additives in the oil will be different. Most cars use a separate gearbox oil and most cars have a dry clutch. So a car oil could make your clutch slip or knacker your gearbox. If you're buying a bike that's been run on car oil, beware!