I used to service my diffs about yearly, but since I drive my truck a lot less now, I go about 2 years between service. Well, it is that time again, and i've noticed some new trends in gear oil.

For the front D44 diff, i've always used 80w-90. That's what the service manual recommends. I notice now though that a lot of people are using 75w-140 in the front instead, and I've read that many dealerships are starting to use that in place of both 80w-90 and 75w-90.

Question is, should I consider using 75w-140 in the front instead of 80w-90, and if so, why?

 

The only reason is it's for severe duty like towing it has shown to make the diff gears run cooler I did it just because I tow a lot

 

If it ain't broke..........

 

I don't see how thicker oil in the front would have any advantage since the gears don't move unless you're in 4x4 and when you are it's normally at low speeds.

 

Ya, that's kinda what I figured. I can sort of see for the rear (especially if you tow/haul a lot), but I have to use 80w-90 conventional (with Ford friction modifier) in the rear because that is what my Auburn requires. Doesn't seem like it would really make any difference in the front though. Probably stick with what i've always done.

 

That's what I would do.

 

I am currently running Lucas conventional 80w-90 in the rear, with the Ford modifier as well (also thanks to my Auburn LSD). Because I run conventional, I change at 15k miles which for me is about every two years.
I run Royal Purple in the front and I honestly can't remember if I used their 75w-90 or 75w-140 in there. I know I had both in my hand for a while at the auto parts store and honestly now can't remember which I ended up choosing. I will change the front at 30k because it's a full synthetic. It's easy enough to remember it that way too, just have to change it every other time I do the rear...