i've got an '03 FX4 and have always used stock fluid. do you guys run royal purple or anything like that?

 

Amsoil severe gear

 

Amsoil severe gear with motorcraft friction modifier if you have a limited slip.

 

Motorcraft, Amsoil, or Royal Purple, all are good stuff. And as mentioned, friction modifier for LSD. Royal Purple claims their oil contains their additive already, safer bet is to get the Motorcraft modifier to add in.

 

I like the Lucas products. I believe 75/90 front and 75/140 in the rear. Use full synthetic...I have heard bad things about royal purple but you know how that goes, one person likes it one hates it...I have used Amsoil as well, another very good product.

John

 

I used Super Tech 75/140 with the Ford friction modifier. By the way, the modifier smells like liquid death!

 

I use good old 75w90 valvoline non-synthetic with my truetrac. Works like a charm.

 

Amsoil w/Amsoil friction modifier.

 

That's the most economical way to do it right.

Yeah, the modifier stinks - we used to call it skunk fluid.

 

I was just going to post a new thread but instead I shall semi-jack this one. About 3 months ago I changed my diff fluid with Royal P and added 4 oz fric mod. I changed it because I was starting to get that dreaded chatter/ rough takeoff from turn/ reverse issue. It still did it so I added three mor oz of fric mod. I wasnt comfortable adding that as the RP was supposed to have it in it already. Problem was still there.

Before I got my new wheels I noticed a speed wobble and it got worse when I got the bigger tires. Took my truck in for the airbag recall and had my dealership spin the tires to make sure. (they were off some by 5oz) still had the speed wobble but now it would go away. Ruled out drive shaft as I could make it not do it, and figured I was going to have to do the rear end. Just for the fun of it I decided to change the fluid and grabbed some Mobile 1 and more Fric Mod. TAAAADAAAAA no more wobble no more rear end drag. Sorry Royal purple, now I wish you werent in my transmission.

 

I run Mobile 1 in my rigs that don't call for using non-synthetic. Once I put the TrueTrac in the 07, per Eaton it is strictly standard rear end oil.

 

Amsoil Severe Gear with the MC friction modifier. I think the oil already has a modifier in it. Anyone know if adding the 4 ounces and using an oil that already has it causing any problems with the LSD?

 

It shouldn't cause a problem - sometimes you even have to add more modifier to stop chatter.

I don't think Amsoil has modifier -

 

Redline 75/140 rear with 2 bottles of FM. RL 75/90 front.

 

You're right, must have been another oil I was looking at back then.