Going to change out the rear differential fluid on my 04 5.4. Going to run Mobil 1 and install a PML cover for the drain/fill plugs and added capacity.

Do I need to put friction modifyer in there as well?? If so, anyone know how much to use? Where to get it?

Thanks.

 

If you have a limited slip diff than you need to run the modifier. If you do, I would add 2 bottles of the Motorcraft friction modifier.

 

Actually I would start with 1 bottle if you have the LS and go from there. I believe the friction modifier breaks the gear oil down faster so I would only put in whats needed. I may be wrong on this but I thought I read that when researching gear oils when I did mine.

 

I was going to change mine too..I have the 3.55 LS, do they make gear lubes with the friction modifier already in it? does motorcraft gear lube already have it?

 

Lucus Oil Synthetic says it has modifyers in it.

I'm going Mobil 1 s it looks like I'll have to get the modifier and add it.

 

Some gear lubes already have it in it. Usually the premium synthetics but check the bottle first. When it comes to gear oil I prefer Amsoil sever gear. I was having trouble with chatter so when I did mine I used the oil and 1 bottle of modifier. No more chatter. I do not think the MC has it in it but the modifier is only 5-6 bucks a bottle.

 

They do, the fords require the synthetic gear oil and most brands already have the modifier in it, but not all of them so just pay attention when you buy it. I know royal purple and valvoline are two brands right off the top of my head that have it already in the oil. It will make note of it on the bottle if it does.

 

Thanks guys! Stupid question, does the front diff require it too?

 

If it's already in your oil I wouldn't add more, but if it's not then look in your owners manual for the specified amount.

 

I know the Lucas Differential oil has the friction modifier, I have a open diff, so I do not know if it has "enough," but overall its been good and was the cheapest at my local autoparts store 39.99 for a gallon (4 quarts). My truck has a mag hytec cover so it requires me to have more oil.