What exactly is the reasoning behind Ford requiring 75W-90 weight gear oils in some of its differentials and 80W-90 weight in others? I haven't really been able to discern any kind of rhyme or reason as to which kind of axle gets what kind of fluid. Even the shop manuals are inconsistent on the subject.

Is there really that big of a difference between the weights? It seems to me that any difference would be negligible, particularly if synthetic fluids or blends are being used.

 

I am no expert but the difference between 75W and 80 W would be hard to detect unless you lived in Nome Alaska.

Now, there might be a difference in specification that affects the additives and limited sliped package, so that might be where the difference lies. 5 points in winter rating is insignificant for most purposes. Additive packaes and specifications are significant. So check the required spec in your manual and check to see if either or both oils meet the spec.

Good Luck,

Jim Henderson

 

I was reading a thread over at Flat Rate Tech that mentioned this subject which inspired my question. An interesting response in the thread was that Ford uses 75W-90 for fuel economy over 80W-90, and that the difference in the weight has little to do with axle load or anything else. That explantion seems consistent with the idea that there really is no discernable difference b/t the weights in 99% of the applications.

Jim, your point on different additives and limited slip packages in the various gear oils on the market is also well taken.

 

performance wise no appreciable difference (IMO) between 75w vs 80w. rockledge, i think you are driving a ranger and I didnt know about the large difference betwwen rear end oil as used in my 97 f1504.6 with 3.55 gears. this f150 requires 75w140 synthetic.

 

Yea, Ford says to use 80W-90 in both of my axles, but I use Valvoline Durablend 75W-90 (synthetic blend) in both with a little bit of friction modifier added to the rear diff (3.73 LS), as well.

I noticed that the guy with the E-150 in the thread I mentioned also had 3.55 gears and it also required 75W-140 just like yours does. Maybe it has to do with the design of the 3.55 rearend?