"synthetic" gear oil and LS additive?

[nthach]

I just did a spill and fill on a friend's 98 Explorer with a 3.73 limited slip diff. Used 3 quarts of 75W-140 and 12 oz of 80-90 to bring the level up as I did buy the last bottle of SuperTech "synthetic" gear oil. The rest was Valvoline SynPower.

The synthetic oil says it's suitable for limited slip diffs but reading on the Explorer and Ranger forums, some said they needed to add in Ford's LS additive. I drove the truck around and I didn't hear any tire scrub or clunking sounds. Should I add some just in case?

 

[d00df00d]

If the oil says it's suitable for LSDs, doesn't that mean it already has the necessary friction modifiers in it?

When people say they had to add Ford's LS additive, did they mean they had to add it to that oil specifically?

 

[exranger06]

No. I have a Ranger with the same limited slip. I've used Valvoline Synpower, didn't add any additional modifier, and it works great.

 

[nthach]

Originally Posted By: d00df00d
If the oil says it's suitable for LSDs, doesn't that mean it already has the necessary friction modifiers in it?

That's my thought too - but I've been reading stories of people who just freshly filled up with new diff oil needing to add the LS additive to it.

When I drove the truck around I didn't hear anything funny. Both the Valvoline and ST said suitable for limited slip differentials.

 

[A_Harman]

Last year I did a diff lube change in my Dodge truck with Mobil 1 75w90 that was labeled LS. But I would get chatter from the diff in low-speed tight turns. I added a bottle of limited slip additive, and the problem went away. On the next fluid change, I used Redline 75w90 in it, and didn't have to add any LS additive to keep it from chattering. Now I stay away from M1 diff lube.

 

[DGXR]

Just tell your friend to drive his Explorer. If he notices the rear end making new noises (chattering, groaning or whatever), then put in some LSD additive. But don't do it yet, there is no evidence it's needed.