After trying everything (except ATF) listed in this thread, I personally have settled on Motul Gearbox Mineral 80/90. Black, stinky, and slick...my right foot tells me that this stuff brings the gear box to nothing short of dreamy. Simple click, click, click through the gears.
Gearbox and differential lubricant Molybdenum bisulphide MOS2 reinforced Extreme pressure.
Specially recommended for noisy or/and loaded gearboxes. All mechanical transmission, synchronised or not synchronised gearboxes, gearbox/differential, transfer gearboxes and hypoïd differentials without limited slip system operating under shocks, heavy loads and low revolution speed or moderate loads and high revolution speed.
API GL4 and GL5 / MIL-L-2105D Extreme pressure lubricant for an efficient anti wear protection, reinforced with Molybdenum bisulphide to handle heavy loads. Stays in 90 grade after KRL 20 hours shear test as requested by SAE J306 Standard, July 1998 update Very high lubricating power which decreases friction and wear. 90 grade at hot temperature to provide outstanding oil film resistance at hot temperature and/or to reduce transmission noise. Suitable for any type of seal and yellow material used in gearboxes design. Anti-corrosion, Anti-foam.
Viscosity grade SAE J306 80W-90
Density at 15°C (59°F) ASTM D1298 0.902
Viscosity at 40°C (104°F) ASTM D445 164 mm²/s
Viscosity at 100°C (212°F) ASTM D445 21.7 mm²/s
Viscosity index VIE ASTM D2270 157
Flash point ASTM D92 198°C / 388°F
Pour point ASTM D97 -24°C / -11°F
To add to the engine oil portion of the discussion...the biggest thing is change it, along with a new filter regularly. Personally I run Valvoline VR1 50W - OR - if I happen to have it on hand Torco V/ST 50W.