I have a 93 F-150 5.0 AOD with the 3:55 rear end. Best I can tell it is not the limited slip because there is no 'l' after the 3. I am trying to determine what fluid to put in the rear. I am changing the truck over to all synthetic fluids. I currently have 10w30 in the engine and will be changing the tranny fluid to synthetic in the near future. I am contemplating putting 75w140 in the rear, similar to the newer rear-ends. My other Choice is 75w90. Any reason why 75w140 would not work? Thanks,

regards,

Bryan

 

I use Mobil 1 75W90 in my front & rear diffs with no troubles . If you are doing a lot of towing or hauling then the higher 140 should be ok . How many miles on the truck ? Just my opinion , but going to a synthetic engine oil with an engine over 30 to 40 K miles & you will likely see an increase in oil consumption . Synthetic is too $$$ to be adding a quart or two between oil changes . I believe if you are going to switch to a synthetic , do it at the first oil change . You wont be gaining anything by switching to a synthetic in a vehicle thats 8 yrs old . Regular oil changes with a quality oil filter will serve you just as well .

 

Just as a response to the comments on the synthetic engine oil. It is quite probable that synthetic oil added in some older engines may cause leaks if the oil was not changed regularly. Problem is caused by the sludge and or deposits that build up around seals when using Dino oil, (conventional motor oil), this build up is a result of heat and combustion by-products. If the oil is not changed often enough this 'stuff' causes the seals to harden. When you introduce the synthetic oil into an engine under these conditions, the synthetic fluid actually cleans away this 'stuff' and seeps past the hardened seals. A good synthetic oil will not burn anywhere near as easily as conventional motor oil. So I don't think you are burning it, I think you may be losing it. Enough about that. My engine has 89,000 miles on it, quite frankly I expected some leaks going to synthetic, but so far there has not been any. So the previous owner must have changed the oil correctly.

My question was more specific to the rear end fluid, what would I gain or lose going to the 75/140 vs 75/90, they both would be synthetic.

 

To my knowledge there is no noteworthy difference between those two weights, or their behaviors in a differential. To be absolutely safe, one would probably go with the Motorcraft 80W-90 if it's available in synthetic, but I've personally heard of others using the heavier stuff without a consequence! Perhaps someone in the oil and lube forum would know the pros and cons?