Im putting a new finned aluminum rear diff cover on soon.My rear diff is a limited slip. Would royal purple 75/140 with friction modifier be right? Can I use the same for the front diff? The truck has 83000 miles, the rear diff fluid is probably close to needing change but should I do the front to or wait more miles?Only use 4/4 in winter maybe 20 times a season...If that matters or is it the age of the oil?How much does the front diff hold?I read the rear is about 3.5 quarts?

 

I used that Valvoline 75W140 w/ the LS additive that comes in it. IMO, I should've used the Motorcraft stuff that doesn't come with the additive already in it. I'm half tempted to get that Valvoline crap out of there and start over.

LS additive won't do anything in the front differential unless you have a LS in there.

 

Is the Motorcraft additive better than Valvoline?

 

IDK. I think I jumped the gun and bought that Valvoline fluid and should've just stuck with Motorcraft diff fluid and friction modifier as recommended. So far I've had a bear of a time getting the rear end to stop locking up around slow, tight corners. It tends to act up after the fluid warms up.

 

So you probably need to add more LS to the diff?
I am going to do this soon,just want to get it right the first time.

 

I guess. I'm going to buy another bottle of Motorcraft friction modifier and give it a shot. If it doesn't work, I'm starting over with all Motorcraft fluids.

 

Keep us posted

 

Rob, I changed my rear out with the 75-140 Royal Purple, its got the anti-slip additive already in it but I still had to add a bit more of the Motorcraft friction moddifier to stop the scuffing around corners thing, been great the last 30,000 miles. I think we can use the same 75-140 now per Ford recommendations for the front diff. somebody will correct me if I'm wrong. Jason, if I had those symtums I'd be swapping out that fluid for the Motorcraft or adding more F.M. to your Vavoline, no sense in ruining your rearend, my .2 cents anyway.

 

Mike, You're right about the gear oil. 75w-140 is OK for both. Nothing wrong with using the one with the additive in the front. It's just not really needed to have it.

 

The additive won't hurt anything up front?

 

There isn't anything in there that the friction modifier will hurt or cause slippage. Just an added expense, of the oil, that isn't neccessary.

 

Thanks for the clarification Roland, good to know about the (don't need) FM up front.

 

Thanks Roland

 

Ok, so you think I should add alittle more fm right off the bat then? What happens with too much fm?

 

Too much fm and your diff won't lock when you need it to. It will be too slippery for the clutches to engage, like on a wet or icy incline.