I found this link fun to read:
https://www.thelubricantstore.com/un...ty-grade-chart
I guess 75W90 gear lube is similar in viscosity to my old 20W50 engine oil, which I ran for 150,000 miles in my 4.3L Vortec engine.

(The Lubricant Store is where I got my Mobile 1 5W50 I ran in the Graphite Crossfire in the summer heat - could not get it any where else.)