Cone style suregrip synthetic gear lube.

I run dyno and a half bottle of that whale-oil. I traded my 4.10s for these used 3.55s, in about 1998, and they have been in near continuous use since, I'm guessing I put over 120,000 on them since 98. I freshened the cones in year 2004ish. I set the turning torque to 120-ish ftlbs using that mix; works real nice. At 130 it tends to act like a spool. At 100 it slipped too easy .You can check this on the car.
I would drop that full-synthetic in a heart beat.

And I tried to run thatchit in my 4-speed with no luck. The brass just would not bite. I took that trans down 3 times in a week, making mods to the brake cones and such. I mean I really tried. I was down to 17 minutes to pull; the trans, the GVod behind it, and dropping the dual 3" pipes; on a 4-post hoist of course.
In the end I installed 50/50 Dextron II and 85/90, and the thing shifted like lightning.
So this cowboy is done with synthetics, at least for EP substitutions.

Like I said, you can check the turning torque of your cone-type, in your driveway. With dyno oil and a half bottle of whale-oil, thoroughly mixed,and at the pinion with one wheel imobilized;
> you need at least 100ftlbs; then your "posi" will work in a straight line. 110 is better.
>If you don't have at least 100 ftlbs, your cone-type needs to get rebuilt.
>If you have over 130 with 1/2 bottle of whale-oil , and it's a streeter, and it consistently squeals the inside wheel on turns; you can add up to 2 bottles of whale oil to get it to behave. If it doesn't, then you need to take it apart and re-shim it.
Yes cone-types can be rebuilt.... but not indefinitely.
I find 120 with my 295/50-15 BFG radial T/As adequate for power sliding. Yet the inside only squeals on sandy turns. Otherwise it differentials nicely