Which weight gear oil?

[Snaketamer]

As the titles states...Which weight gear oil? 80w90 or 75w140? I'm going with synthetic and friction modifier. TIA

Greg

 

[Red98CobraStang]

im assuming this is for the differential, and you want to use 75w90. 80w90 is Ok to use if you have to, but never use 75w140 in our differentials. It is WAY WAY WAY to thick. Id use 75w90 royal purple max gear oil. Its only a few bucks a quart at NAPA and it already has friction modifier in it.
Grant