I have a 97 F150 4x4 with 355 LS rear end. The manual states to use 80w90 rear diff oil. So went to dealer got the friction modifier and Motorcraft Premium 80w90 diff oil. OK I'm loosening the bolts to the cover and the oil is draining and I get to a bolt that has a tag on it and it says to use 75w140 synthetic oil. Well I had it apart and no other vehicle so I put the 80w90 in. Will this be ok? Should I change to synthetic? Thanks for the feedback.

 

You better do it again!!!!! I`m getting ready to do my Rear End Diff. too, but I read the tag first. My local dealer told to ONLY put in it what the tag says to use. Ans its not cheap either, I couldn`t get it in my area . I had to go back to the dealer & man!!!!! $15.78 a qt. you need 3 qt.`s to have enough plus the modifier too. Its really your call on this, but for piece of mind ,I`m using what the tag calls for. I hope this helps you .
Lariat#1

 

Yep, I'd put in the correct weight. Don't know what would happen, but I would not want to find out if you left the 90 in. Besides, there has to be a reason to spec for "synthetic 75w140". I think what you read was for the FRONT axle. I thought it was 75W90 however.....????

Anyway, I changed my diff fluids about 4 months ago and found that Amsoil from GI Joes had the same specs that the Ford stuff did for only $12.99/qt for the 140 and $8.99/qt for the 90. The dealer wanted $19.95/15.95 respectively........ I got the Amsoil stuff, but did buy the friction modifier from Ford for only $4.95.

So far so good. I tow a #4500 TT as well too.

 

I got Royal Purple synthetic from my Local NAPA for about $8/qt.

You dont have to put in the friction modifier with it. It's complete.

I even got a gasket for the cover too at NAPA.

 

I work at Valvoline, so i can get synthetic gear oil for 3 bucks a quart!! I could get the regular stuff for free, but i want the synthetic.

Ford differentials and transfer cases dont need to be changed until 100k miles. Thats why the differentials dont have drain plugs, its not supposed to be done very often.

- Rick -

 

same thing happened to me.went to get an oil change and they recommended replacing the diff fluid.so I said go ahead and next thing you know when I'm turning corners my rear end was kinda like jerking.turns out they put regular diff fluid in and not the 75/140.I posted a thread here a long time ago and got mixed responses.What I did was add friction modifier to the regular diff and ordered some of the 75/140 from amsoil.As it stands now,the regular diff fluid w/friction modifier is still in there and no problems.I am going to change it to the synthetic once it warms up.So what I would do is add the friction modifier asap and wait to change it or go ahead and change it back over to the synthetic.You can order the 75/140 through amsoil like i did.Hope that helps.

 

Thanks for all the replies. I got the 75w140 synthetic from the dealer ( couldn't find it elsewhere) and more friction modifier and changed it yesterday. Three quarts of oil and friction modifier came to 59.00. Not cheap. I should be good now for quite awhile as the diff gears looked fine from what I could tell when I drained it the first time.

 

ive heard that ford spec gear oil wont absorb water properly so some techs say not to use it. Instead i stashed mobil synthetic 75w90 with fords frition modifierer. Seems to be ok so far. 17k miles. Worse comes to worse ill drop the 3.55 and get some 4.10's in there.

 

I have heard that a lot of dealers are putting 80w-90 in the rears because the 75w-140 won't absorb moisture. I had the people who did my gears dump the 80w-90 in my axle when they did the swap for that reason. May want to check with your dealer, first, though.

-Flea

 

I work at a dealer and all we use is 80w90 regular gear oil and friction modifier if it is LS. We had alot of trouble with rear ends when we put in 75w140. Since we switched to 80w90 all the problems went away. My truck is leased, and I might drain out the synthetic and put 80w90 in. I just don't trust that other stuff.

 

Well the 75w140 synthetic was ok in my diff for 83000 miles and almost 7 years so I gotta think that it must be pretty good lube. I have never had any problems with the rearend in my truck.

 

Hopefully you never will have any problems. Don't get me wrong rear end problems are not the norm. The majority of trucks never have problems, but we do see a lot more than we used too, since the switch to 75w140.

 

Maybe the 75w140 has nothing to do with it, could be it's the way the rearends are being manufactured nowadays. Maybe Ford is using a different supplier to cut costs and then you have so so quality. Like I said earlier 83000 miles and 7 years on 75w140 worked for me.

 

I have also heard some people say that bearing quality is not what it used to be. Who knows?

 

So you are telling me not to trust what ford put on their rear end when they manufactured it?? Should there not be a recall then to change that tag that says 75w140 if it is destroying rear ends? What other tags should i not trust on there?