Also, I don't want to rain on your parade, but I have the same axle and it takes 75w140.

75w90 is for the front differentials on 4x4 for trucks if I'm not mistaken.

I am about to change mine this weekend

 

Eh well I'll add the additive, I went with w90 per suggestion from a friend. Too expensive to frig around with new oil.

 

My truck requires 75w140 for the rear also, and 75w90 for the front. I had a guy who owns a fleet repair shop tell me he uses the 90 weight in everything and suggested I use it for my truck rear. I told him it wasn't an option because there's just too much difference in weight.

I refreshed his memory Thursday about that conversation when I was picking up a case of Royal Purple rear axle gear lube that I ordered from him (75w140), and he said he wanted to retract his previous comment about it being ok to use the 90 weight for my truck (rear). He didn't go into detail but his words were, "you were right, it is not a good idea to put that lighter weight oil in the rear of a Ford, it can mess things up if you tow anything."

I didn't ask him to qualify that comment, but apparently either he messed someone's truck up or he spoke with someone that did it with an unfavorable outcome. I don't know.

 

I also went per this article, note the red ** and read the ** notes.

http://www.fordf150.net/howto/differentialfluid.php

 

Definately add the modifyer.It will prob be fine with that weight oil,hard to say though.only time will tell
I myself am not a big fan of valvoline but its not my truck

 

That's interesting. Anyone know if this is true or false? The fluid in mine was almost certainly original with >90k miles on it and everything looked to be in good condition inside.

One thing about the Motorcraft synthetic 75W140 rear axle lube, it's pricey. $15/qt at the local Napa. The 75w90 I used in the front diff was about $4.50/qt. And 4 oz. of LS friction modifier was $4.83.

 

I paid $11 a qt for the 75w90 at NAPA.......

Everything looked ok to me too but......it was 10yr old fluid.......

 

Here at the Autozone, even Mobil 1 gear lube is very pricey, just like the motorcraft....roughly $18/quart, or more.

Continental Motor Sports (sells Redline) is 5 minutes north of me. Applegates is 5 minutes south of me (sells Royal Purple), and I order Amsoil frequently. All of them sell their axle lube for $8/quart. If I pay cash to the RP guy, he does not charge me any tax. So it is hard to pass that up.

Even Walmart's brand of synthetic axle lube is $9.66/quart here.

Both the Amsoil and Redline modifier is about $4.50/4oz.

 

Well just an update I got the modifier stuff, suctioned out some of the fluid in the diff, adminstered 4oz to the diff and refilled with the fluid I took out until it was full. Does everything sound alright now? I also did some figure 8's in the local resort parking lot....probably thought I was a lost tourist haha.

 

You should be okay now. About how many miles did you drive without the modifier?

 

Under 100, probably around 50-60 After adding the modifyer, no leaks, squeaks or cheeps! :-D

 

From what I can find, if your rear LS axle is a Ford or Dana 80, the use of 75W-140 synthetic plus the LS additive is required. If it's a Dana EX 80, you may use 75W-90 or 80W-90 and synthetic is not required - but the additive is.

You may use any additive that meets Ford spec EST-M2C118-A, 4 ounces.