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· Registered
67 Posts
Discussion Starter · #1 ·
I am considering putting synthetic gear oils in both differentials and transfer case (NP241) on my 90RC .318 (55K) 4WD with AT, 9.25 axle, 3.5 axle ratio and limited slip in the rear end. Live in North Florida and tow a boat average size 20 foot boat. there is no information in the owner's manual and FSM on this subject (synthetics weren't invented yet for this application?)
1. What viscosity should I use in each differential and differential case?
2. Is Mobil one the way to go with each?
3. After draining the oil in each compartment should I replace the gaskets as well?
4. Should I use the limited slip additive in the rear differential with the synthetic?

The reason I am considering the change is that I understand synthetics have a higher temperature before breakdown occurs. Is this just hype and really not necessary? I have no problems with the vehicle-this is just a preventive maintenance thing.

Thanks for all the help!!! ;D

· Super Moderator
19,859 Posts
1- I have been using Mobil 1 75w-90 in all of my vehicles diffs,
Use Mobil 1 Synthetic ATF in your t-case.
2- Mobil 1 is good stuff, some will say Amsoil or Redline is better (I would doubt they are much better), but Mobil's availability is a plus.
3- On your difs, remove your old gaskets, clean it up and just use RTV without the gaskets.
4- Yes, unless you use an oil that states you don't need the additive (I believe Amsoil does not require an additive).

Synthetics are good, the hype (to a point) is justified. It is superior in basically every aspect except intial price, but if you factor in the longer drain interval, this is a moot point.
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