I am changing the rear differential fluid in my 2001 F150 w/LS. Ford specifies 75w-140 synthetic and with a friction modifier. I see 75w-140 synthetic both "plain" and "for LS". I assume the only difference is the presence of the modifier (ie., it would not be necessary to make any add).

Is that correct, or is there another difference? My reason for asking is that I would like to use the plain oil and add less of the modifier in order to get the maximum limited slip.

 

I'd just buy the LS and forget about it if it works. BTW, I had to add some extra additive to the LS oil because it wasn't enough.

 

i used Amsoil, which claims you don't need to add the LS additive...i added it anyway, never had a problem.

 

My understanding is that the additive reduces chatter and grabbing by lowering the limited slip action. In other words, moving towards a conventional open differential. So if there is too much additive, there would be no obvious problem, but there would be little or no lock-up.

Did you decide you needed more because it was still chattering?...and under what conditions? (I'd like to have max limited slip even if it sometimes chattered a little).

Can anyone confirm that the difference between 75w-140 synthetic and 75w-140 LS synthetic is just the presence of the friction modifier?

 

yeh, its made for THE limited slip.

id use the LS, and not even think twice about it.
your asking for trouble if u run the wrong stuff...

 

What if you don't have a limited slip rear end? Would it hurt to put the LS kind in if that's all you have at your nearest store?

Well, I know it wouldn't hurt, but would there be any negatives to it?

 

Not, unless the LS variety costs more.