I raced Spridgets for many, many years and we just always used the stock o-ring, they are cheap and readily available. I've found the secret to sealing the axles are to use a new oring, Permatex Ultra Black RTV (gasket or no gasket, I never used them, and mount the wheel back on the car with all the lug tourque to get a good 360 seal, and get all the gear oil away form the area as you renew it all, I always drained my rear end dope, used brak clean to flood the bearing dry, then starting sealing it all up. Hope this helps.
I doubt there was any great secret to what your friend was doing, he probably just found a o ring that was like the stock one, and it was easier for him to get, a thicker or thinner o ring wouldn't help in anyway. Hope this helps.