While each product has its own unique viscosity with in a range, in general 75w90 is about the same weight as a 10w30 motor oil as this chart shows.Not quite that simple, but let me confess that I didn't do enough checking, either. I should have checked my formulas with Wikipedia, which is quite detailed and complex.
I thought I was working off the kinesthetic viscosity, but ...mistakes were made. Let's just say that gear oil is just about always considerably "thicker" than motor oil.
Given your suggestion to quote my references, I have decided that I did it wrong.
This comes from Amsoil btw.