Rear Diff Flud
 

Change the fluid in my differentials a couple weekends ago. I used Mobile 1 Synthetic 75W-140LS in my rear diff. Said it has the friction modifier in it and I haven't noticed anything odd or out of the ordinary after changing it. Had some people claim you still have to add friction modifier or it will screw up the rear end. Any truth to that?

You should be ok if it had modifier in it. The Amsoil is the same way. I personally use it in my truck's but I still add some modifier anyway. It won't hurt to add additional modifier.

If it isn't chattering there is no reason to add any.

Adding more will make the limited slip unit less effective.

From 50k miles on, I have uses the Mobil 1 Synthetic 75W-140LS also. Never have added extra friction modifier to it. 194k miles now, no issues.

Thanks everybody. I was pretty sure this was the case but just needed some confirmation from more knowledgeable people.

How much does it take to change? I'm needing to change mine and want to buy the upgraded Ford diff cover with the cooling fins

3.5 quarts or so. Got 4 and had some left over.

I used the Valvoline oil for limited slip. Every thing was fine until I started pulling this trailer around the country. I had to add 1 bottle of fords additive. Just thought I should pass this along.

Ugh this thread just reminded me I need to do this. Been putting it off because I wanna upgrade the cover at the same time. Money and time, two things there's never enough of!

I upgraded the diff cover several years ago. The new capacity is 6 quarts. I use Royal Purple 75W-140 and two years ago added AR9100. That took care of a chattering problem I had.

Ed

I have the Ford aluminum diff cover and it took less than 4 quarts so I'm sure it depends on which one you have.

No matter the cover, the bottom of the fill port in the diff is the correct amount for coverage.

At 100k I ent no additive as y limited slip was limited no more. Didn't change by not using the additive. At some point I'll pull the clutches and replace.

I just did mine. Got the aluminum cover, put a black paint job on it, with the stainless allen head bolts. Used Amsoil with most of a bottle of friction modifier, but the point i wanted to make is i also used a Lube Locker gasket which saved me from having to use permatex and waiting 24hrs before i could add the oil back. No leaks so far.

If you're going through the trouble of opening up the differential, you might as well toss a true trac in there and have a traction device that actually works and then never worry about friction modifier ever again.

Do you have one in yours,it doing ok on the street or towing heavy, im installing clutches,side gears,spider and gears in mine now but with 340k guess its time.