1989 420 radiator cap aftermarket?

[Bishop]

Last week the water level dash sensor said the water level was low while on a trip. I stop the MB to check the water level, and the seal on the radiator cap broke.
I put it back in and headed for a parts store, and the guy handed me a aftermarket cap. It fit and we were back on our way.

My question is the new Cap says 20lbs on it. I had to have one at the time, is 20 the correct pressure for the 126 body on the 1989 420SEL. Should I get another from MB store?
Thanks