I dont know how much difference between the oil will make if they are of same grade.
I think what the difference may be, is that with motor oils propensity to airate, foam, and bubble.Fork oils do not. They have to allow room for release of that compressed gas from those bubbles, in a sealed chamber. In a hot climate, it may be worce? It's not an issue when motor oil is used in a motor, because of the crank vent.I know when I changed my stock oil it had light colored apearence, as if frothed.
So when fork oil is used, a higher volume can be installed? Less likely to foam,and build pressure? The added benifits are maybe the need to check and replace your fork oil less often? Also maybe less likley to pop a fork seal? Around here, motor oil costs about 3-4 bucks a QT. Fork oil about 7-8 bucks a Pt. Now if I'm gonna go through all the effort of pulling that front end apart, I would spend the few extra bucks for the better product.
If you use fork oil or motor oil, will it change the rotation of the earth, no. Both are probably just fine.