I have a 2006 F250 6.0 CC SB Lariat and was wondering if there is a way to tell if it has the limited slip without taking the cover off. I'm going to change the fluid need to know if I need to get the friction modifier or not.
Thanks.

 

On the plate inside the drivers door it will say axl, example, 3.73 = 3 or 4.10 = 4. If it has a capital L beside it, it's a limited slip. It's on the bottom of the sticker, bought in the middle.

 

Mine shows 3L so makes it a 3.73 limited slip. Thanks.

 

Yup. Make sure you get the oil without the friction modifier and have the modifier separate. Add half first, see how she runs for a it before adding the rest.

 

Good luck finding it without the friction modifier in it. When I was searching I could only find Royal purple with, Mobil 1 with unless I bought the motorcraft from the dealer.

Later found that Carquest had the best prices local to me.

 

Ugh...all they had was mobile one with the modifier. I don't have a chatter problem, just wanted to change out the fluid. Will it hurt to use it?

 

No, most use that. But walmart sells it without the modifier.

 

Click on "vehicle" and put in your VIN, it will tell you everything about your truck:
FordEtis home page

 

I have read many threads that state synthetic LS fluid is good to us in our differentials. LS meaning it has the modifiers already in it.

Many here use Mobile 1 synthetic LS fluid, I used Valoline Synthetic LS and have not had an issue. Am I missing something here?

https://www.ford-trucks.com/forums/9...id-change.html

 

The only reason people say to get fluid without the friction modifier is so you can "tune" LS to work better by only adding maybe one quarter or one half of the reccommended amount.

The more friction modifier you add, the more the LS clutches slip. Only add enough to stop the rear from chattering when turning.

A lot of the factory LS units are unlimited slip like mine was. I purchased my truck used, so I don't know the full history. When I changed the fluid in mine, I used Redline without LS fluid and didn't add any friction modifier. It still wouldn't grab, nor did it chatter. Last Feb I put a Detroit trutrac in the back. Now I do have a LS rear.

 

Ok, so your just managing the amount of the modifier being added. I'm with you now.

Thanks,

 

I have the Sterling LS 3.73 rear differential on my '03 F250. I bought Valvoline Synpower 75-140 which includes some friction modifier. I called the Valvoline folks and asked them whether I needed the additive. They said I needed half the amount called for by Ford as they have LS additive in their product. They said their product does not require additional additives for any other vehicles except Ford does need one bottle. I bought one small bottle at the Ford house and added it to the Valvoline. Everything worked perfectly. Your owners manual discusses the details of the lubricant in the rear differential.