Oil type

What oil are you guys running? 5-20 or 5-30?
Also when I was changing the oil pan, ( I bought the motor from a local builder, ( used about 30k on it) I looked up and saw a new piston. I have no clue why I only have one new piston, from the looks of it, it's never been fired with the new piston. Do I need to run and additive, extra zinc, or any special oil for the first start up?
Any opinions appreciated. Thx