Again and again this tripe crops up, don't suppose you feed your cat dog food do you.
Seriously motorcycle oil is at its most basic effectively engine AND gearbox oil mixed together. A car's engine oil has a high compressive tolerance to be squeezed into gaps, it doesent have a high shear resistance needed to keep it on the face of meshing gears.
Also there are the different additives needed between the engines. Cars have friction reducers as said, bikes also have friction reducers (ha wasn't expecting that) its just car oil mainly uses silicon based friction modifiers. Bikes use cleverer chemical modifiers that don't affect clutch opperation.
By comparison car engines are junk, specific output of 120HP from 1600cc, thats 75hp/litre! my 250 makes 50hp, or 200hp/litre. Thats an old smoker, the big bruisers are at or above the 180HP/L mark.
If you want to chance it use car oil, in an old shed i prob would too but think on this, my old 600 had 30K when i sold it. I did 29K of those on fully synth bike oil changed every 3K and never needed ANY of the reccomended part/service replacements. The bike is approaching 40K now with its current owner, still with good oil as i won't fit anything else (i do the servicing). It has the original timing chain and clutch, and has needed only a couple of new shims. With the fact the bike was tuned in my ownership and is making 15hp more than stock is even more impressive, in 2001 i lapped the TT in under 30mins (god knows how the racers do it in 20) so it was used properly.
fin