Need a little help please. I have a 2004 F150 4x2 Supercrew. My right rear seal is leaking and also now have a slight leak from the cover of the differential gear case gasket. I just bought 4 quarts of 75w-140 Mobil 1 Synthetic Gear Lunricant LS which states it has limited slip friction modifier included. My question is....will I still need to add 4 ounces of limited slip friction modifier when adding the new oil? I bought some just in case, but wasn't sure if it is needed due to the Mobil 1 oil indicating it's already included. Thought I would go ahead and replace both seals while I have the gear case open and changing the fluid. It's my first time tackling a project like this, so any feedback will be appreciated. Thanks for your input and help.

 

You probably won't need the FM but you might.

I put valvoline 75w140 synthertic in mine and it claimed to have the FM also. It works okay and there is no noise coming from the clutches when I turn but it locks up really quick. I think I need to add a bottle of the Ford FM to mine.

 

If the FM you bought isn't Ford/Motorcraft, take it back.

 

So will it matter if I'm mixing Ford friction modifier with Mobil 1 oil, or does it also have to be Motorcraft brand oil? Thanks.

 

You can mix it.

 

You don't need friction modifier if oil already has it. I used red line in my sports car that has limited slip for years and no issues.

 

So some mixed messages here....some say add the friction modifier even if the oil states it has it included. Some say that if already in the oil, then there is no need to add the friction modifier. So, my question is....if the oil supposedly already has it mixes in, would it hurt anything to still add 4 ounces of friction modifier? Going to tackle the job this weekend. Thanks for your input.

 

I would not add the FM unless it needs it and you won't know that until you try it without the addtitional FM

 

If it chatters after a week, add Ford modifier.

 

Just put the Ford FM in with the lube that says it's already in there... Did that for about 4 or 5 rear diff fluid changed in my 9.75" LS rear end in my 97 F150 and I used different lube each time..

The LS still worked great even after 255,000 miles on the truck and kinda miss a good ol LS rear end...

I've got the electronic lock rear end, but if I don't engage the EL, it's just an open rear end... And then, even if I engage it, it only stay engaged up to around 35 mph... WTF...

Good luck!

Mitch

 

Thanks gang. I will post how the project turns out. I've worked around different vehicles from time to time and try to do most things myself instead of taking to the shop, but never have done this project before where I have to remove the axels to just change out oil seals. Appreciate everyone's help!

 

Do yourself a favor and buy a rear cover gasket instead of trying to use the silicon gasket stuff.
Remember that you have to push the axles in a little to be able to remove the c-clips on the axle ends. It's a pretty straight forward job.

 

Wanted to give an update: had a chance to complete the job on my truck this past weekend. Only had one bad axle seal, but decided to replace both while I had it torn apart....figured it was just a matter of time before the other side would start leaking. I also noticed that the pinion seal was leaking to, so I replaced that as well. Overall a pretty straight forward job as was mentioned earlier. Took me a lot longer than it should have, but some of that time was traveling to return wrong parts for proper parts. Also, I did not add any friction modifier. The oil that I used already had it mixed in. So far the results are great. Truck is running great and no more leaks. Thanks to everyone who gave feedback and tips!!