20psi at 1500 RPM is NOT that low. its below average for a new crate engine but many people have built engines with this pressure or less. I'm tempted to think he reused the oil pump and a worn oil pump will have such a poor seal that it will have little pressure at low speeds- the other option is that he is just running larger clearances, and at one time that was a VERY common thing to do.
I wouldn't go too much thicker and I'd give serious thought to buying a better pump (m155hv from melling is about $40 without the pickup, which is about another $10). You may have other issues (leaky pick up, clogged pick up, leaking oil main, etc.) but it is possible for everything to be fine and just have low oil pressure if you have large rod, and main, or lifter, or cam clearances.