This is straight from hydro gear, I emailed them back in the spring asking about oils for the zt5400 in the cheetah's. We recommend 20W50 standard engine oil. Any reputable brand(Pennzoil, Valvoline, Mobil etc) will work. Synthetic can be used but it really doesn’t provide much of a benefit over the conventional oil. The service manual says api SL but they did not mention anything about it when I emailed them. Be careful if you remove the vent plugs or drain plugs, they will strip the trans housing out if you over tighten them. Torque spec is 32–42-inch pounds on them. They hold around 5 quarts each of oil.