Which oil pump
Try 10W-30 oil. I've seen noticable differences when trying different viscosities. They also flow faster on initial start-up. Straight weight oils take more HP than multi-weights. Even with a 10w-30 in my engine, it can go way over 100 when revved dead cold.
Not sure of the clearances my engine builder put in, but I use Amsoil 5w-20 Dominator series with a standard volume/standard pressure pump.
BTW, if you are concerned about zinc content, Amsoil has several with lots in it!
https://www.amsoil.com/dealer/techservicesbulletin/MotorOil/TSB MO-2007-08-08 Flat Tappet.pdf
Amsoil source & info:
www.thelubepage.com