Just got off the phone w/ customer service at Royal Purple. The technician said that Ford specs 75w-140 for my rear differential, but that he recommended I use 75w-90 for better gas mileage. What's the consensus out there for the best one to use? Sorry if this has been discussed at length. Tried a search and didn't really find what I was looking for. Thanks.


2004, 5.4, 4x4.

 

I used 75w-140 in my truck with no problems but I do think I remember hearing something about Ford spec changing to 75w-90. I'm not completely positive though. If anything just run 75w-140, you'll be fine.

 

The spec change was the other way around. In 04 the recommended that the rear take 75w90. They later changed the spec to 75w140.

 

Gotcha, I knew it was something.

 

Haha, you don't know anything!

 

Haha.

 

Better gas mileage? What, like 0.2 mpg?

I'm more inclined to believe the folks who design and build these axles than a company out to sell oil.