Brian: My generic service manual says 50:1 TC-W3 would be OK. Lower unit uses Quicksilver Premium Blend Gear Lube. It is water cooled by an impeller type water pump. As to the year, it is listed as 1977-1989. The significance of the ser. no. would be mfg. date. There are different service specs according to the ser. no. Example: prior to 6A1-000100 or 6A1-011601 etc... Lynn. PS: It was built by Brunswick Marine Power (Force, Mariner, Mercury and MerCruiser.)