I'm changing the gear oil in the rearend of my 67 mustang. What weight gear oil should I use? 80w-90 or 85w-140. I live in Ohio to give an idea of the climate. The 80w-90 is more for the colder areas. Is that what I should use?

 

You are correct. You want the 80w-90 weight gear oil in that environment. 90 weight is just as good.