The late Dave Munday (a well known Cortez mechanic) always used 90wt gear oil. (specifically Schaeffer 209a moly gear oil).
Milo, adding the street ell is easy. Buy a 3/4 street ell and cap from a good automotive supply (I bought mine from a truck parts supplier). Replace the side filler plug with the street ell and cap (pointing up). Because it raises the fill level, it makes the reservoir hold more oil. Also makes filling easier.