I have my old Eaton series sixteen outdrive on my 1966 16 Ski Sporter and yes it still runs fine. The manual calls out for "a first grade, heavy duty gear oil, SAE 90 weight, containing Sulphur-Chlorine-Lead additive - or Eaton p/n L-100"

Anyone have some idea of what to use these days? I don't mind paying for whatever is the best.