I tried to find this in the threads with no luck. Someone posted a question if you need limited slip additive if you use Royal Purple in rear diff, I never heard an answer and my new diff cover came yesterday. I plan on doing it this weekend. Any info would be appreciated.

 

Sorry found my own answer:

Max-Gear® is recommended for use in truck, motor home / RV, and automotive front or rear differentials, manual transmissions and lower gear units of marine engines that specify use of an API GL-5 or GL-4 fluid.

Max-Gear® is an ultra-tough, high performance, GL-5 automotive gear oil designed to provide maximum protection to heavily loaded gears while maximizing power output throughout the drive train. It outperforms other GL-5 gear oils because it combines the highest quality synthetic oils with Royal Purple's proprietary Synslide® additive technology.

Max-Gear® makes gears run smoother, quieter, cooler and longer without overhauls.

All viscosities of Max-Gear® are formulated with hypoid friction modifiers necessary for use in clutch or cone type differentials. No additional additives are necessary

 

My gears never hardly see a load on them...wonder if I could go to the 75W-90W Royal Purple

 

I don't realy understand the heavier gear oil in the rear vesus the front unless it is load. Or the fact that when ever the truck is moving the rear is always engaged and the front only when in 4WD and at slower speeds. Price is the same anyway.

 

i just picked up the royalpurple for the front, rear, and transfer case. i also got the 8oz. of ford racing friction modifier. holy crap!!!! it was $112. and that was no gaskets. i just read the above posts. should i return the friction modifier?

 

Yes return it. I have RP rear fuild and it works great in my LSD. No need to Waist the Friction Mod.

 

CCerniglia...do you use the 75-90 or the 75-140??

 

Does it meet C8AZ-19B546-A -- EST-M2C118-A, right out of the Ford spec sheet for the truck. If it does then use it and happy driving.

 

I too had my gears done, the stock LS rebuilt and am running Royal Purple. I have the additive just in case I need it, but so far, my gears are whiny but not chattering.

So Royal Purple's mix is good with the stock unit.

I believe it is mostly the Detroit and Auburn that require a special additive above what a high quality hypoid has in it.

 

75w140, Its what is Ford says should be in there, i wouldn't dare risk a new Diff over trying a new fluid.

 

I have a supply of the Amsoil "Severe Gear" 75w-110..I was hoping to give that a try

 

I run the RP 75w90 in my F-150 diffs. I run this truck HARD in the mud (200' passes in 7.6 seconds), and have no problems. I have a LS in the front and a stock LS in the rear, no additional modifiers needed.

 

I can tell you that i don't see alot of Rear Diff jobs ever comes through the shop on a 10.25" or 10.50". So i know FoMoCo got something right with that Rear end and i'm not going to change a thing. What was the 75-110 from?

 

I got it from a dealer friend

 

We are talking about a much bigger axle. Next are you going to say because you run 5W20 in your truck, that the OP should run it in his 7.3L? There are reasons why the motor company’s use different fluids in different applications. He should stick with the Recommended Diff, Trans, and oil Grades.