been wondering this as well as my old F150 (2010) called for 75w140 but these new 9.75s call for the slide of 75w85. Is there a reason for this? I do understand how those weights work, based on the temperature the oil may never even reach that 140 weight. Just wondering either way. I have the Spicer cover and lube locker gasket in my inventory for a change at 50K miles, I’m at 40K now.
A 50°F difference is pretty big, how high were the temps getting? I took some temp measurements last year on a 70°F day after my truck had been sitting for 5 hours, I'll post the results below. I should have waited longer for the ECT and ATF to come down to ambient temp, but I was impatient.
I’m in northern Indiana
My tests aren't apples to apples to yours, and I didn't document my data as well. Keep in mind my readings were after my diff was repaired, but working properly. The highest I saw was on a 79f day after nearly a full 36 gallon tank was used at 72mph towing. It was 203f at the hot point on the camera on the back side adjacent to the cover. Basically right next to the carrier bearings on the cast metal. IR cameras suffer from all sorts of inaccurate readings on various surfaces, so take it with a grain of salt. The cover was only in the 190s f. I do know I changed the fluid 3 times in 6 months, and the last fill was the specified fluid and it was dramatically lower, and the ambient temp was at least 5f hotter when I used the camera the last time. The hot spot was in the low 150f much like your data. Same truck, trailer, camera, location on the diff. My, possibly incorrect, conclusion is the "140" fluid is too thick. Maybe I'm wrong, and the thicker stuff protects better even though it gets hotter.
In my observation that is close enough in desired weight. I do not believe it would threaten a warranty.
