i have a 97 f150 with a 9" rear with about 84k and it has a tag that says 75w140 syth only and priced around for oil and its around 16 to 19 bucks a qrt do i need the friction modifyer as far as i know oil has never been changed what is a god oil to use in it any input is appreciated!

 

i use 75w140 full syn in my 97 3.55 non limited slip. But plenty of people say the much cheaper to buy 75w90 weight conventional works well in those rear ends. Apparently ford went to the 75w140 syn just to help meet epa improvements. Not in my office right now and i dont remember the code for limited slip. you can look up your vehicle id/door jamb labels on internet to determine if you have limited slip. You only need the friction modifier ifyou have the limited slip rearend. I use "stalube" brand from orileys with good results. Also, some people say the rear end fluid never needs changed unless you rebuild it or submerge it. I changed mine at 100,000 miles and also at 150,000 miles. So far at 165,000 miles zero problems.

 

You do not have a 9" axle. Ford has not made those since the 1980's. It is an 8.8, completely different design. Since the factory fill was synthetic, unless you have submerged it, there is no reason to change it yet. By the time you get 100,000 miles on it, 75W-140 may be more commonly available and cheaper because that is what GM is now also using as factory fill.

Jim

 

Also, the 75W-140 would increase your mileage as compared to the 75W-90, so it can't be an EPA related issue. From what I've read, the 75W-140 is especially required if you have the towing package. For example on 4x4's, 75W-90 for the front and 75W-140 for the rear. I've heard about changing it out after getting submerged, but doesn't the rearend have a one way valve on it?

 

I currently have Coastals gear oil in both of my differentials, with the right amounts of L/S additive in both diffs, little extra in the front. Gear oil stands up pretty well if its rated correctly. Its not like it takes the abuse engine oil does. If you never have the water submerssion issue, go with a high dollar oil for the long run. Yes, you need to switch out the oil if it gets submerged. I have found uncomfotable amounts of water in the oil after goin wheelin. Since I do change the fluid a couple times of year due to deeper water, I dont worry about big expensive synthetic issues. Coastal isnt a bad product in either respect.

 

my front diff is a 8.8 and my rear is a 9.?? so a 9. i just changed my front diff with castrol syntec 75w90 full synth and the oil that came out was kinda milky so i asme water was at fault in the rear i am considering mobil 75w140 full syn any comments on this are greatly appreciated

 

Ford is presently experiancing problems with some of their rear ends. The kit they are supplying us with to replace axels that are wearing out (where bearing runs on axel) comes with 75W140. This is an upgrade for the faulty axels. It is full synthetic. Must be a better lubricant for the bearings and that's why they recommend it. I don't think it is for the gears themselves just the axels.