The first question is do I even need it? I read the manual and it said nothing about it. the sticker on my door jam has the code H9 which I am almost positive means I have a ls rearend. The next question is how much do I add and where can I buy it? thanks the truck is a 2003 FX4 F150

 

You add one 4 ounce bottle when you change the differential lube. You buy it at the Ford dealer. If you have a LS, yes, you NEED it. H9 is a LS. 75W-140 synthetic gear lube is also required.

 

should have asked earlier but that 4 oz is a complete flush of the rear end oil right? Anyone know how much the gasket for the cover costs?

 

4 oz modifier is for a complete fluid replacement. The cover does not use a gasket, you use silicone sealer, also available at the Ford dealer. Some people have had good luck with generic sealer, but others have had leaks. It's probably more important to prepare the surface and apply it correctly than to use a specific product, I believe the stuff at Ford is made by Permatex.

 

Read this caption:

 

thanks for the input guess I will have to make a trip to the ford dealer and pick this stuff up.

 

Steve's instructions applies to your truck except you use synthetic 75W140, not standard 80W90. Everything else is the same.

 

The 4 ounces of friction modifier is in addition to the rest of the gear oil that you have to put in there.

 

no way of knowing how much I would need to add if I didn't drain the whole thing though?

 

Throwing the whole bottle in won't hurt it. However, why not drain the whole thing if you are going to do anything at all?

 

Didn't really want to spend more money than I had to right now. Do I have to pull the cover off to drain the whole thing? sorry I am not used to working on my own truck. Another thing I have noticed is that I have a whirring noise coming from my truck. I thought it was the rearend because it has been seeping for a while, But it sounds more like it is coming from the front of the truck now, any ideas?

 

If you are not draining and refilling the whole diff, why are you even asking about friction modifier?

 

If you are not planning to drain and refill do not bother doing it at all. How many miles do you have?

 

I was worried that the fluid was getting low and I was just going to add some until I could afford to take it apart and reseal it the proper way. I am going to eventually drain it and reseal it so it doesn't seep out anymore. I asked about friction modifer because I was worried I would harm the truck by just dumping some gear lube in there if I added any. I have 52,000 miles on the truck

 

If you feel it start to chatter, then throw some in. It's more important to use the correct grade of lube.