Cone style suregrip synthetic gear lube.

68383GTS said
Yes there is a little room. I have read the cone style 489 suregrip is made by Auburn and you are not suppose to use synthetic gear lube. Also read if you add to much friction additive they will act like a open rear end. Not sure if this is correct or not?

68383GTS said
So does anybody know for sure if I will have to change out the synthetic gear lube? It is not a new unit.

I can’t tell you for sure that you should, but I can tell you I would.
Some say you don’t need the modifier at all with synthetic, but you see for yourself what can happen if you don’t.
Some also suggest using a thicker oil.
After doing a lot of reading from places that do diffs the consensus was to use 90wt dino oil with the modifier, and this was the overall I got from it.
Thicker fluid will not as easily get into the cones or clutches causing a lack of lubrication and overheating of the surfaces.
That made total sense to me.
I used Valvoline 90wt and one bottle of the modifier and had some chattering on corners as well as a bit of shudder when the sure grip let go going into a corner, or coming out of one under load.
Then I added another bottle of modifier and everything smoothed out and the unit seems to work just fine.