installing chrome diff cover and purpule cranium rock crusher cover. owners manual says 75w90 but i bought 75w140 royal purple. will that be too thick, also do i still need to add an additive, its says it is already in the fluid. i have not drained it yet so i can return it. thanx for the help

 

I'm sorry I can't help you but it seems like you should get what the owner's manual suggests.

But, lets get some pictures of the cover and guard!

 

only put the recomended weight in. That tick oil won't get into all the small areas it needs to as easily and you can burn up your rear end. Someone said that they put the friction modifier in even though royal purple has it. Is yours a limited slip axle? If not you don't need the friction modifier

 

75W90 is being phased out for use in most axles and is being replaced with 75W140. You'll be fine.

 

Quintin

Can you check out my thread, ATTN: Ford Techs

Thanks!

 

I've been running Royal purple 75w-140 for a while. Seems fine to me.

 

in my gto i ran the royal purple and did not need the friction modifier it already has it in it. not sore of the wight tho i would think what ever ford suggests u should use.

 

When I ask someone in the parts department at the dealer what I should run in my diff, I got the same answer.

I had just change it and put in Mobil 1 75W90. He said I would be fine until the next change.

 

Just curious, why is the 75w90 being phased out for the 75w140? Does the front differential take the same fluid?

 

The unofficial reason is to help gearsets run a little quieter. The official reasoning is to add that much more protection to the bearings and axles.

(and I ain't forgot about your other thread, gimme a few)

 

RP 75w140 is the way to go. I've had it for about a year, and all is well.

 

My dealer replaced the 75w90 with the 75w140 to fix some shudder and a rolling howl the diff had at low speeds. The diff has never run so quiet. That was 6 months ago and all is well.

 

Did it fix the shudder?

 

I did have AMS oil in the diff after the gear change.. but the gears had a louder whine than they do now.

 

Yep! Well thicker oil and two bottles of friction modifier.