Both Tuff Torq and Hydro Gear recommend 15-50 or 20w50 full synthetic API-certified oil for their sealed units. Personally, I use Mobil 1, but any brand-name oil, such as Pennzoil Platinum, Rotella, etc., is fine. Wheel Horse, Cub Cadet, early Bolens, etc., with the Sunstrand or Eaton pumps, use...