Right heres my stab at it then!
Cars compared to bikes demand long service intervals so the oil has to be designed for this. The oils therefore have friction-modifiers and other additives in to keep them working for a longer time
If you use car oil in your bike, the friction-modifiers cause a problem because most motorbikes have a wet clutch.
Car oil gets onto the clutch, the friction-modifiers get to work and you'll end up with a slipping clutch.
Bike oil doesnt have friction-modifiers, so you dont get this problem. Im pretty sure but oils for wet clutches have an MA label on it.
Another thing to consider, is that car oils are not designed for a gearbox. The meshing of the gears shears the oil which does not happen with a lovely smooth crank. Which causes car oil to foam, losing its lubrication.
Motorbike temperature flucuates a lot quicker than a car. Larger mass of the car engine combined with the larger amount of fluids and a sealed engine bay mean the temp changes are less pronounced. The higher power output of a bike engine compard to size means higher temps with less fluids and metal to absorb the temp change, causes rapid change in temp during operation. Which means the oil needs a better temp tolerance to cope with this. Finally car oils tend to be very thin and will burn off and some may get past the rings and burn off.
And on top of all this a motorbike revs 4 times higher than cars.
This site gives good info regarding oils
http://www.chris-longhurst.com/carbibles/i...eoil_bible.html