I asked Lucas oils about adding friction modifiers to their dino 80/90 gear oil for a limited slip diff. This was their response ...what do you think?
Our 80w/90 has a friction modifier package already in the formula. I would
also use some of the Heavy Duty Oil Stabilizer in with the gear oil around
10% of the capacity. The Oil Stabilizer will keep the oil climbing up the
gear and keeping the oil temperature down in the differential.