Gear oil has high pressure additives that are not allowed to be in motor oil. So you really can't compare the two.I know this is an older thread, but it's still out here so thought additional info might be helpful because people still read this:
Even though it it gear oil and it can be used, the 75W-90 (and yes, someone already pointed out they use different scales for gear vs engine oils) actually has a lower viscosity than 20w-50, so probably not as good as using synthetic 20w-50.
The 75w-140 is thicker (almost 50%), and may make things a little quieter, but with the downside that it actually retains heat a little more and will keep things slightly hotter, while not adding any additional wear protection due to similar moly and other additive levels, so there's your trade-off. Heat vs noise. I live in the southeastern US, and with the heat, I'm sticking with 20w-50 or SAE60 depending on season.
Past that, at 200f they all pour like water.