Just turned 7500 miles and I could not wait til 15000 for the differential change. Changed fluid with Mobil 1 75-90 syn. First time up the gravel driveway, noticed something new. No wheel spin or gravel hop! (I have an extremely steep gravel drive going up to my house) Got out of the driveway into the the field beside the house and lo and behold, went through in two wheel drive when 4wd had always been required in the past. I did several stop and starts looking at at the side mirrors and sure enough, the limited slip worked like a charm! Is there something in the factory fill like assembly lube that will make the limited slip perform poorly?

 

"Is there something in the factory fill like assembly lube that will make the limited slip perform poorly?"

Maybe it's the same stuff that makes it look so bad after 15k miles - mine looked like black snot. I too used Mobil 1, and after the next 15k miles it looked like a darker olive oil.

 

I took my truck on a 15 mile drive before chainging the fluid. The old fluid was greenish gray and smelled like rotten onions. Never smelled rear end fluid that had that kind of smell. Inspected the gears and everything looked good. No metal flakes in the old fluid. I even used about a pint of new Mobil 1 flushing the old fluid out and didn't find anyflakes or anything that looked like metal. The cover was coated with a fine black substance that I figured was wear metal. I am glad I did the change just for the limited slip. I couldn't believe the change it made!

 

I just did my pmi cover yesterday, the old fluid had a bit of aeration, but it was not dirty & no metal on the magnet. I used schaeffer 75/140. for the refill. I also did the transfer case. I have been running gear oil in my 96 transfer case for the last 150,000 miles. I dont believe in using trans fluid in the transfer case. Gear oil keeps things much quieter

 

doug60, do you have the Dana or AAM differentials in your truck. Both my 2005 AAM differentials did not have magnets on the inside of the cover plates. Actually, I never found a magnet any place in either differential. The factory lube oil in my differentials was changed at between 1400 and 1500 miles or 2303 km. The old lube was cloudy creamy to milky and it looked like water drops were first escaping out of the bottom of the diff gaskets as I loosened the lower bolts first. There was a black film of what looked like soot covering the inside of the diff covers, case interior and gears. I sprayed everything with varsol three times over a few days and blew it out with compressed air. My truck is not a daily driver and a couple weeks later I refilled the differentials with Amsoil 75W-90 severe gear lube. I sure did not like the way that factory gear lube looked.

 

Just did mine today, before the truck quit, which is another story, but found like Fay the oil looked foamy with a greenish tint and the covers were coated with a blackish residue that wiped of with a rag. Hope the Mobil 1 synthetic works better than the factory stuff. Front cover was a little difficult to get to the top bolts but other wise very easy to do.

 

Its the AAM , and its does have a magnet in the inspection / fill plug.

 

I changed my front and rear today. Both had the mag's on the inside of the fill plugs - AAM 11.5 in the rear. My stock fluid looked great after putting 14,000 miles on it. I guess you can never till what your fluid will look like until you crack 'er open and take a look!

Until I bought my current truck (1st Dodge CTD) I had been a long time Ford diesel truck customer dateing back to 1988. If I didn't use a Limited Slip Friction Modifier the RE's would howl and grab when you turned. I guess this isn't a concern with the Dodge's or at least I haven't found any post on this site.

 

If you dont tow that much would runnin 75w140 hurt anything?
I am comin up on my first diff change but havent gotten around to puttin my gneck hitch in, but I would rather run 140 than 90 if I could just to be on the safe side when I do start towin a lot.
I realize that usually the 140 is more expensive and was wondering if that is why people said to run 90 if you didnt tow or if 140 would hurt a truck that is not loaded.
thanks