I’ve used API GL-5 rated in my early Olds, no issues. However, I did a search and found on the Currie website they state GL-6 for their 9 inch or you void the warranty. See below from their site:
9-Inch API GL-6 Requirement
Due to the high-offset hypoid design (very high shear) of the 9-inch ring and pinion gears, a severe duty petroleum-based gear oil with API GL-6 specification is mandatory. The use of synthetic or GL-5 specification gear oils will result in premature wear of the gears and excess heat buildup that will damage internal components, voiding the Currie Warranty.
Also from the Currie site: ALTERNATIVE GEAR OILS THAT ARE PROVEN TO WORK IN 9-INCH REARENDS
Torco - RGO 85w-140
SWEPCO - 203 MolyXP Gear Lube 250w (Racing or extreme application)
And from Mark Williams Enterprises here in Colorado:
4) FILL REAR END ASSEMBLE WITH OIL. FOR DRAG RACING M-W RECOMMENDS TORCO 85-140 MINERAL BASED OIL.
For me, next time I change the diff oil, I'll use an API GL-6, probably the Torco 85w-140 mineral, no synthetic. Alan