Your 1970 XLCH has basicly two places to put oil... The engine (in the oil tank) and the primary/transmission. The primary and transmission share the same oil. When you put oil in the primary, you are also putting oil in the transmission.
You normally want to put the same oil (20w50) in both the engine and the prim/trans. This is because there is a oil transfer valve that will allow some oil from the prim/trans to go into the engine. This happens if you put too much oil in the prim/trans. Its a safety, so you can't overflow the prim/trans... The excess will just go in the engine and end up in your oil tank. Harley did not want too much oil in the prim/trans because of the dry clutch used in there untill 1971. Harley removed the oil transfer valve in 1977.
Some people remove and plug the oil transfer valve so they can run different oil in the prim/trans. And I guess it works for them. But I don't recommend it. I still have mine. I find it is very handy. I never have to check my oil in the prim/trans. Every now and then I just dump some in there... I know I can't overflow it.