I did a my diff fluid yesterday. What a freaking difference. My girlfriend got mad at me cause she thought I bought new tires or something cause it rides that smooth now. When I popped the cover off the fluid was actually pretty good. I don't they added the friction modifier to it, and most likely the wrong weight was in it. I had my teacher check it out and he said it looked perfect. So if you get close to 150,000 miles change it out! I used Mobil 1, and it takes 3 quarts. I only had 2 Mobil 1 quarts. So I had to buy a Valvoline one which I am a huge Valvoline fan, and its like 9 bucks cheaper a quart. So go valvoline, oh and use only Motorcraft friction modifier. Had a lot of my teachers talking about it yesterday.

 

I will be changing all fluids in my truck every 50,000 miles. Im going to full synthetic when I get there. Im at 6000 miles now and have changed my oil twice!

 

good plan there. I changed my fluids at 30k and again at 90k.

 

On our Rear Ends with Limited Slip all you can use is 75-140 which only comes in full synthetic.

 

Don't forget Wal-Mart Supertech 75W140 GL5 Full Synthetic, $10.46/Qt.
I use it in all three of my vehicles: '97 F150 XLT, '94 Lincoln TC, '94 Ranger 4.0 XLT Ex Cab. All three rides have the 8.8" non-locking differential.

 

I replaced mine the other day because the seal around the cover started leaking. The fluid that was in there looked pretty good but I put royal purple back in there.

 

Wal-Mart dosen't carry the weight we need in Texas