The sure grip additive should work fine with synthetic. FWIW I use the GM spec stuff sold by AZ with both synthetic and standard lube.

If you are running older seals in the rear, it may prefer conventional lube. Synthetic is nice for really cold running (eg below zero) or continuous hard running hot (eg. road race using xxW/140).

If you are not doing the above, suggest standard 80W/90 plus your additive to reduce chance of leaks... and save your money.