So, I recently bit the bullet and ordered a 9" rear end kit from QuickPerformance for my '78 Malibu. Along with it I received a card saying it requires 80W-90 GL5 non-synthetic gear oil with Posi additive. I was wondering if anyone that has used a swap kit, or just have owned a 9" in the past, which gear oil you would recommend? Is there gear oil out there with additive already in it? Or do I have to buy the oil and additive separately? Any brands I should consider over others?