They have Sta-Lube 85W-90 GL4 at NAPA and also on Amazon.com. Has been working great for me. I asked Brewer's and they said they would recommend I use this.

http://www.amazon.com/Sta-Lube-SL24239-GL-4-Multi-Purpose-Hypoid/dp/B000M8RYMC


'70 Duster 470hp 340/T56 Magnum/8 3/4 3.23 Sure-Grip