The only real additive that motorcycle oil has that car oil doesn't is zinc. For the wet clutch. Which is the reason car oil isn't good for bikes, the friction reducers make the wet clutch slip. I use motorcycle oil in my bikes, but only to top it up when the level gets low. Because 20W-50 motorcycle oil is all that I usually have in my garage, being I have two bikes. And, btw, been running 20W-50 Valvoline Max Life in my Ranger since 2002. Has 166,000 miles on it. Has never been a problem in the winter, even on those rare days we get close to or slightly below zero. And yeah, 20W-50 is great for making the engine quieter.
Know what's funny? The Valvoline oil change place down the street used to put 20W-50 in my Ranger all the time. Never questioned me on it. The customer is always right, correct? Last time I took it there, two yeara ago, they flat refused. Told me it's a 5W-20 engine. I called him a moron, told him it was a 10W-30 engine, that's what the oil cap used to say. And the owners manual had 10W-40 and 20W-50 in it as options in the temperature chart, said 20W-50 is still good down to 20 below zero. They refused. I told him, "you just lost a customer forever." I'll change my own oil from this point on. ASSHAT! I do believe they went "Green" on me.