The original oil pressure sender is set to show the oil light on when you hit 15psi.

If you showing 20psi at idle...not really a worry maybe your tick over is a bit low?

The main part is 2000rpm as Mike points out MUST be min 35psi.

My 2 Cossies have newish engines in the Escos has a high pressure pump and the Saff a std one.

Escos @ idle at 1000rpm tickover is 35 (will drop to 25 after a really hard thrash but its only running semi synthetic oil at the mo)...at cold its 70psi ish warm at 3000rpm 55psi

Saff @ idle at 700rpm on crap running in oil is 30psi ish on cold about 55psi and at 3000rpm around 50psi