I'm in the process of changing my rear diff fluid. I have a 2006 5.4L 4x4 with 3.55 limited slip and the 9.75" Rear end. The owner's manual calls for 2.7 Quarts of 75W-140 full sythentic. I put in a 4 oz. tube of CRC Friction Modifier and 3 full Quarts of Royal Purple 75W-140 Full Synthetic with Friction Modifier. Also, I used VersaChem High Temp (Red) RTV silicone for the new gasket. That's all fine (and hopefully helpful to anyone who searches for diff fluid questions).

Onto my question. I know it can't hurt to put a little extra modifier in there, but I put 3 full Quarts in and it didn't overflow from the fill plug? Should I go buy another bottle and fill it until it overflows or will I be good? On past trucks, I'd just fill it up until it would overflow from the fill plug. I just want to be sure before I go driving anywhere.

 

If you over fill it, it will just leak out the vent tube when it get's hot. Put in what the book says and let it ride.

 

Yeah, that's what I always did in the past, but that was on older trucks. I didn't know if it was ok or whatever on the newer ones. Thanks a lot

 

I accidentally miss took quarts for pints and WAAAYYYY over filled it. Stupid move. The only issue I had was diff fluid dripping from the underside of the bed everyday for a month. Really stunk up the garage.

 

RP doesn't need an additional friction modifier.

 

I wasn't going to put any additional in with the royal purple, but people on here and outside the forum said it doesn't have enough in it. Think it will hurt anything putting 4 oz. of CRC friction modifier in with the royal purple?

 

Does the front differential also have a vent pipe/tube? I changed out my fluid 6 weeks ago, but after one week I noticed a very small amount of fluid clinging to the bottom of the casing. sitting in the garage it would never 'drip' onto the floor, but there would always be a "oil stain" on the bottom; just enough to pick up with a clean cloth.

I thought I had a very small leak, but would this be caused by a vent pipe?

 

"I put in a 4 oz. tube of CRC Friction Modifier and 3 full Quarts of Royal Purple 75W-140 Full Synthetic with Friction Modifier."
Are you sure?
If you need to spend your money put more modifier in. It won't hurt or help.

"...after one week I noticed a very small amount of fluid clinging to the bottom of the casing."
I put a Mag-Hytec cover on the rear dif. It is the only dry pumpkin I've ever had on any truck that I've owned.

 

I could be wrong, but it just seems to me the 9.75 rear gear holds 3.75 qts.

 

Well I took it for a drive with the 4 oz. of friction modifier and the 3 qts of Royal Purple. The gears don't chatter and the limited slip doesn't seem to slip any more or less than it did before, so I'm guessing all is well..It seems like they do hold more than 2.7 qts like the book says, but hey..I'm a believer in the book. They didn't chose that quantity for ****s and giggles

My front diff was changed probably 10,000 miles ago, and the bottom of the diff/cover seems wet with fluid also. I checked it and its not low, so I don't worry about it...I just check the level everytime I change the oil.