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I think manual calls for Mobil GX90, BP SAE 90 EP or Esso 90/140. Which has same viscosity at room temperature as motor-oil. But don't get their functions mixed up. Gear oil is much, much better for gearbox than motor oil.
Gearbox doesn't work on hydrodynamic wedge pressure-film like with engine-bearings and pressurised pump, so gears actually touch each other. Gear-oil has lots more boundary-layer extreme-pressure/extreme-heat additives such as zinc & phosphorous for contact lubrication. Contact area of metals are like two mountain-ridges (one inverted) sliding & grinding against each other. When these tips touch, intense heat & pressure is developed. EP/ET additives melts under such conditions and generates a tough wear-resistant ceramic-like coating at that spot to prevent further wear.
Moly is another lubricant and functions as small ball-bearings between surfaces. Boron is also used to enhance wear protection.
A lot of Triumph owners like Lucas or Red Line synthetic 75w-90 gear oil. Lighter weight might help lubricate output shaft sleeve bearing. If you've got noisy trans due to extra wear from previous motor-oil use, might want to try Red Line's Shockproof line of gear oils, really helped soften whining rear transaxle on my track-car when no other oil could.
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