In the next couple of weeks I'm planing on changing most if not all the fluids in my truck. I have a 2002 f-150 4x4 with a non limited slip rear end. I recently read the how to on f-150.net and noticed that the person who did the write up used 3qts. of Redline 100% synth. Okay not a problem I figured that I would just go pick up the same stuff. Well here's the problem none of the auto parts stores carry Redline. So here are my options Valvoline, Lucas , Lakewood, and Royal Purple. I personally am leaning twards the lucas oil but since this will be the first time doing this I want to make sure that I put a good quality oil in it. Also I been told 2 different types either 75w-90 or 85w-140. Any help would be greatly appreciated.

 

Read page 289 of your owners manual. Front diff 3.7 pints of 75W90. Rear diff 5.5 pints of 75W140 synthetic. If you do not have one go here and down load. https://www.fleet.ford.com/maintenan...ls/default.asp

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Okay so now I know 75w-140. I would like to know why the dealership said I needed 75w-90.
Another question then owners manual says it doesn't need replaced unless it's leaking(it's not) or if the rear end has been submerged in water (it hasn't). So since neither of those apply (even though the dealership said I need it) should I do it anyhow?
In my first post I put pints (pts ) but meant qts (quarts)

 

75W90 in front and 17W140 in the rear. Changing oil before its time will do no harm nor will it do any good. The old spec for the rear diff used to be 85W140. For some reason ford changed it. My paper manual calls for 85W140 and the online manual calls for 75W140. If you check the tags on the axle you will see what was recommended when it left the factory.

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I thought there was a friction modifier that had to be added to it also, no?

 

Only for limited slip rear-ends. The OP doesn't have a limited slip.