Ok here goes. rearend cover was leaking for awhile. So i went to my local parts store and got some rear oil for my truck and some grey silicone and decided to fix the leak today.
Found out that it has some piece of crap chrome cover, if I had known I would have bought a stock one.

I have a 2000 f150 xlt with the 5.4 in it and its a 4x4.
the oil i had bought for the rear is from ford and its 140w 75 and its synthetic and its what the owners manual says to use.

now the question is... Do I need to add friction modifier to it or is it already in the oil, the synthetic oil that I bought. I just dont want to mess up my rear over a stupid leak that I just fixed.

Your thoughts will be a great help!

truck has 120,000 miles on it. bought it used 7,000 miles ago

 

You need to add the friction modifier and I would use the motorcraft brand.

 

how long can i drive around until I get the modifier?

 

You can drive around without it, depending on driving habits, mileage, and wearing. But, is it smart to do it without modifier? NO.

 

The modifier should be installed before the lube.