Gearbox oil

I recently renewed the gearbox oil on my Manual/overdrive 1975 Triumph 2.5PI. I asked Triumph Spares in Fairfield as to what oil they use in gearboxes. Richard said 80W/90 oil that was GL4 and GL 5 compatible. REPCO had a 30% off sale so I bought Castrol 80W-90 oil that had GL4/5 . Richard said that it had to have the GL4 component. Further investigation showed that Penrite make a 75W-85 grade gearbox oil that is dedicated GL-4. This would be the oil to buy. The REPCO store at Hoppers Crossing didn't hold this oil. The gearbox was drained of oil and before adding to the gearbox I mixed some oil with the Nulon Teflon gearbox treatment. I have added the Nulon Gearbox treatment to many of my gearboxes over the last 30 years. I used to use Slick 50 but don't know where to get that now. The Nulon treatment is also excellent in the diff. The Nulon is perfect to use with all overdrive gearboxes.