I have a 1994 f150 4x4, I want to service the differentials. Do I need a friction modifier additive in these?

 

Depends on if you have limited slip axles or not. What are your axle codes on the VIN plate on the driver's door post?

 

Friction modifier makes the oil more slippery which means less grip from the clutches in a Trak-Lok diff, it is used to produce smoother operation on the street
It is possible to have a factory LS diff in the front axle but not likely, most trucks have open diffs. In the rear axle the LS option is more common and the axle code on the drivers door pillar will have a 2 character code with a letter in it instead of two numbers... so H9 instead of 19 for example for an LS diff and 3.55 gears. If the axle code has 3 characters then the front axle has an LS diff as well.

 

I will look when I get home on the door for the code. Just curious would it hurt if It was added but not needed?

 

Only thing it will hurt is your wallet.

I do not use it on my clutch style LSDs, never felt the need...besides that, any clutch style past 100k barely works anyway with normal driving cycle (say 50% freeway, 50% city).

 

That’s for sure. If I recall, last time I bought some it was almost $20 a bottle. No reason to pay for something you don’t need. I would get a bottle to have on hand just in case the door tag doesn’t match what you actually find once you get the cover off.

 

Back in the day, the GM stuff (I worked at a Chevy-Olds dealer) was made with moose fat. We called it "moose juice".

 

Some gear lube contains modifier so it will not hurt anything to use it with an open differential. I tried it when I rebuilt the Trac-Loc on my old 1997 Mustang GT. I restacked the clutches and shims to make it much more aggressive. I ended up using some more modifier to "loosen" it up a bit. Even after another 50k miles that differential was still tight and liked to spin both tires on tighter turns at the end of driveway.

 

Wow! never knew this about the front diff, so my H9B, means LS front and rear?

 

I looked and mine shows axle 19
that’s the only number in that area

 

19 means you have 3.55 open diffs. You do not need friction modifier, assuming your axles are original to the truck.