SAE 20 oil is thin, while SAE 50 is thick. Thin oil will allow quick reaction of the piston, while thicker oil will reduce the bouncing of the piston on rough roads & on hot days, but will reduce the speed that the piston can react to throttle opening, & this can affect the performance, particularly when the weather is cold.

I used SAE 30 oil in mine because
1. I always had some oil left over from the last oil change
2. I am too miserable to go & actually buy some 20 weight oil.
3. I prefered to dampen the piston movement a bit more to help stabilise the mixture better while operating at highway cruise on what we laughingly called "highways" I did a lot of mileage on rural roads back then.

Even in deep winter, i used the 30 weight & drove steadily untill the whole engine was fully warmed up.

The way i remember it, many people also used Auto trans oil. Dexron was popular from memory.