Since 1967, when I bought my '64 TR4, I've used the factory recommended GBX O/D lubricant; 80w90 gl-4 with excellent results. For more detail about this see: http://www.buckeyetriumphs.org/technical/Gearbox/GearboxLubricant/GearboxLub. htm brgds, Jay '64 Triumph TR4 since '67 - now at VTR/TRfest in San Luis Obispo CA