I have browsed many threads and thought I would just post question to clarify. I have the vibration while turning so im going to start with changing the rear diff fluid. I picked up some royal purple 75w-140. It says it has modifier in so do i need to add additional modifier and if so 4 or 8 ozs? Mileage is 97,000 with no maintenance history. Thanks for any help

 

No LS additive needed for royal purple... It's good stuff!

 

Any rear diff that uses an FM ( like MC Xl-3), needs fluid changes every 3 years or 30K or so. FM is hydrophyllic - and the moisture can contribute to corrosion.

good luck
MGD

PS - it if still chatters, you need to add a modifier.

 

And if the modifier doesn't stop the vibration... clutch packs


But others have had success replacing. Hopefully that's the case for you as well.

 

Add at least one bottle of modifier, if it still chatters add a second bottle of XL-3. If it chatters after that you can keep adding but mostly will need new clutch packs. Every brand that says the modifier is in it, still add the mc XL-3.

 

^^^^ don't agree

 

don't have too, but 99% of the time I have used any brand like amsoil or valvoline that states no modifier required, we end up having to put at least one bottle in. Never used RP but I would recommend using the MC modifier.

 

Why wouldn't you use the modifier? Honest question. I called valvoline to ask about it when I did mine and they recommend adding an extra 4oz of modifier for our trucks. That's when using the synpower synthetic with limited slip additive. Unless you went crazy with the amount it wouldn't hurt anything regardless.

 

I didn't mean don't use it , I'm simply saying you don't need to with royal purple and to just keep adding it is a waste IMO. If you switch to royal purple and it doesn't help there is something wrong.
By all means though, if a guy wanted to add modifier it's not going to hurt anything.

 

Most guys add the friction modifer still, no matter what oil. I know i do...

I change my oil every 30k, And i still have chatter. I just ignore it. Dealer replaced clutch packs early on. I just ignore it now. Change fluid every 30k.