John Deere 420 oil pressure

Fresh rebuilt motor started up and had no oil pressure to the head. Took back to machine shop cause they put cam bushings in wrong Fired up after having installed properly and have very little oil going through line to head but have 20 psi at gauge. How much oil pressure should it have?
Have you removed the oil filter canister and checked that the ports are clean.

I have a faint recollection about this slowing oil flow....
No I havent but I will. New filter was installed when rebuilt. Do you know what the oil pressure should be on these? And how much oil flow should there be to head.
Back cam bushing has to be lined up to let engine oil come out to go up line to the rocker arms.. Pressure would be about the same as your operating oil pressure??
Ya the machine shop put the cam bushing in wrong the first time after starting it and had no oil what so ever coming out the line so I brought back to them. Now can see some oil coming out line when unhooked at the head but not 20 psi like the gauge is reading.

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