The factory recommended EP90. I used Castrol EP90 this from 1977-1990 with no issues. Once I tried FUCHS LIGHT GEAR OIL - SAE 80W but on long runs it seemed to weep oil from the filler cap, and I felt the viscosity was too low. I now always use AMS SVG severe Gear 75W-90. I have been riding the bike since new in 1977 and have never had gearbox problems. I change the oil every 5000 miles, as per the owners manual.