Alright, I just got a set of Richmond 4.10's and I'm getting them installed on Friday. The shop said that they'd supply the installation kit, but I have to buy the oil and Posi-additive. They're doing it for $135 which I consider a steal because the installation kits normally run about $75 alone. Anyway... Since I have to supply the gear oil and posi-additive, which is the best? I was told to use 80W-90.. Also, which posi-additive should I get?