Like Adrio wrote, I'll put in my comments. For the first 80,350 miles, I always used straight 30 weight non-detergent oil in my gearbox with overdrive. I had to change the lower lay shaft 3 times. When I restored my TR3A from 1987 to 1990, I asked Ken Gillanders from Temple City in California for the latest on oil for a gearbox with overdrive. He told me to use 20W50 Valvoline R (for Racing) oil. I have driven for 15 years and more than 87,000 miles and like Keoke, I have had no problems. I change the oil every 3000 miles and clean out the strainer in the bottom of the O/D.
Don Elliott, Original Owner, 1958 TR3A