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www.royalpurple.com/product/max_gear/Royal Purple Max Gear 75W-140 is a non-corrosive lubricant you can use in the transmission and rear axle. It meets the GL-4 and GL-5 specifications. I use it on my '51 Stepdown. It was recommended in either this or the member forum a while back. The overdrive uses the same oil as the transmission.