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Differential Service

I’m getting ready to change the fluid in the differential on my 2001 Ram 1500 with the 5.2L engine with close to 100K miles. This will be the first time I’m doing it since acquiring the truck. Upon looking in the Haynes manual, it states I need to put 8 oz of friction modifier while doing the service. I know it takes 2.5 to 3.0 quarts of gear oil to do the job. Upon reading the label on the friction modifier tubes, it states that a 4 oz tube of friction modifier is good for up to four (4) quarts of gear lube. Should I just stick with the full 8 oz like the Haynes manual states? That would be the info I would be more inclined to go with. Any suggestions?

Also while re-assembling the cover, do I put RTV on both sides of the gasket or do I just stick to the Diff. side for the RTV sealant?

When I do that job on my Ford truck I use the product Ford recommends, which I bought at a Ford dealership. There’s more than one friction modifier, and they don’t all have the same specs. Suggest you use the exact product Dodge recommends (by Dodge part number or name/band/etc) and then you can just use the amount suggested in the owner’s manual. Aftermarket friction modifiers might work just as well, but unless you have a reason to use an aftermarket version, best to stick with the one and amount that Dodge recommends.

If it hasn’t been overly long since the last time the job was done, you might be able to just pump it out with a transfer pump, rather than taking the cover off. When I take the cover off I smear a little gasket sealer on both sides as I recall. It’s a little bit awkward of a job. One of the posters here recommended a spray on product for flimsy gaskets like that.

Follow the directions on the friction modifier.

If the diff acts up with just the 4 oz of friction modifier, you can always add more friction modifier.

Put the RTV on the diff cover.

It’s easier to remove an old gasket from the diff cover on the bench than trying to remove it from the diff under the vehicle.


I did mine, different make, $4 extra for 2 quarts from the dealer, after oil and whatever additive, but no muss no fuss. The right stuff it was worth it to me.

Follow the directions of the modifier you have in hand. 4 ounces of the stuff you have is fine. Actually, I use synthetic gear lube and usually don’t add any modifier as the stuff I buy says it is in the bottle already.

Use RTV on the cover only. It is way easier to put a bead of RTV on the cover on the bench than laying under the truck and applying it to the rear. Just have a couple of bolts ready to hold the cover and place it as accurately as you can so you don’t have to move it around too much.

The Haynes manual says to use 90 wt. Napa has 80-90 which is the closest to 90 I seen. They did have 75-90 synthetic which is what I wanted to get. Does it really make that much of a difference? By the way, they gave me the wrong gasket (figures)

90 weight gear oil is essentially the same as 75w90 when hot. The difference is that that when cold, the 75w90 acts like a 75 weight and the 90 weight is very thick. I would go for the 75w90 as an improvement over 90 weight.

I have a 1998 Ram 1500 4WD and neither the rear diff nor the front diff use a gasket. Clean off the old silicone material well on both halves and simply replace it with new when you reinstall it.

Factory fill on the 1500 is 75W90 for fuel economy.
No gasket is needed, you can use RTV, that is how it was sealed from the factory.
The 9 1/4 rear axle requires 5 ounces of friction modifier (P/N 04318060AC) If equipped with limited-slip differential.

Amazon sells 85w90 GL5 from Liquid Moly, if you’re interested in using what the FSM recommends. But as others posted, they’re both 90 weight when hot.

What I will go out of the way for is GL4 80w90 for the tranny in my old Celica (GL5 hurts the synchros they say). But I digress…

I swapped the 80-90 for Valvoline 75W-90 synthetic. I also have O’reilys chasing down the correct gasket so should know in about an hour. I figure this with a 4 oz bottle of friction modifier and I should be ready to go. I’ll update after I get this done in the morning before the sun comes out and starts cooking. My driveway faces west so I try to get any work done before noon.

Casper, see my earlier post, there is no gasket on these, only RTV.

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The last Mopar rear ends that I opened had no gasket, only RTV and Dodge specified reassembling with RTV only.

The service is complete. I did have the correct gasket. Maybe in the old days they didn’t use a gasket but Rockauto lists one & that’s where I got it. All went smooth and I went ahead and changed the transmission filter & fluid. Looks like I’m good for another 30K miles. Cheers


Good job, glad everything worked out as planned. Back on the road with a purring machine.