Originally Posted By: deeter16317
I can only tell you that after 300+ miles of highway, 4.10s, and only a 1500 pound trailer, the rear only registered 136*F. Keep in mind that's with almost 9 quarts of oil and an aluminum cover. I haven't ever measured the rear after towing heavy because they are typically only short trips.
I don't think there is a recommended fluid for "best lockup". I know it took a load of about 10k to finally break my LSD loose and cause it to function correctly. I have only had it function correctly with RP 85w140, and when it applies, it applies fully. Keep in mind these apply harder the more load they are subjected to...until the tires slip.
Also, the 75w90 specification comes from AAM, not Dodge. From what I have read, they have indicated to guys that have contacted them, that 75w90 will function acceptably in 99% of the truck applications.
I agree. If you are not seeing hi temps, there is no good reason to run a 140 grade. But you are only going to know that with a diff oil temp gauge. If you tow heavy and don't have one, the safest bet is to run the heavier oil, I suppose. Also, the 11.5 AAM is offered with the gear type limited slip, as well as the plate type. I know from my own testing that these type of diffs do not increase the diff oil temp substantially. . Even a plate type shouldn't much going straight down the road.
I'll climb on the soapbox here and recommend that anyone who tows regularly should have a diff temp gauge. AutoMeter and Isspro both make them and the Mag Hytec finned cover (another great addition) is plumbed for a gauge. Towing at or near the truck's maximum , especially uphill or against headwinds (or too fast ( : < ) can l generate high gear oil temps. That said, the 11.5 AAM is known for being a cool runner, relatively speaking.