Cone style suregrip synthetic gear lube.

If you have a cone type, with a recipe of Non-Synthetic EP gear oil AND the Modifier, which is correct, and it slips.... Then it needs to be tested for turning torque.
If it is less than about 100 ftlbs, then it probably needs to come out for inspection, and if nothing else, tightened up.
If it is between 100 and say 110, then it may just have too much modifier in it.
If it is between 110 and 120, I use the modifier as a tuning aid.
if it is over 120 with skinny tires, it will act like a spool most of the time..
The turning torque that your unit needs, will depend on your power-level, what trans you are running, what tires you are running, and your driving style. Furthermore; in my experience, MORE modifier is not necessarily a good thing.
For example, I have a stout 360, a manual trans, 3.55s, and 295/50-15s. I like to Second-gear slide around corners about as often as I think I can get away with. My "posi" has to work, else stupid and or embarrasing chit happens. My combo likes 120 ftlbs turning torque, and to get that after internally tightening her up, she only needed a half a bottle of that oh-so-stinky whale-oil, for tuning.