1986 350 mercruiser that was just 100% rebuilt by a local shop - new bearings/crank/rebuilt heads/etc.... I took the scarab out on the lake with a mechanical oil pressure gage and it is bouncing from 60psi (WOT) slowly to 55psi, then jumps down to 45-47psi and works back up to around 55psi... any ideas?? It has the stock windage tray and a new pump, and 6 qts in the pan/filter. Before I pull the motor again I am wondering if anyone has any ideas.... please help!

Check your gauge first. I pulled an engine once and it was the gauge. Worked fine beforehand but not after.

I put a mechanical snap on pressure gage to it.... I don't think the gage is the issue.... anyone have any other ideas

check youre oil level sounds like youre aerating the oil , try dropping level 1 qt . rob

thanks, i'll give that a try..... so how many qts should a 5.7L mercrusier take? 5/5.5/6?

Is water getting in the oil? A milkshake in the engine will cause the oil pressure to behave this way. Check under the oil filler cap just.

My initial thought is that the pickup fell off the oil pump, but most of the marine engines I`ve torn down have a brace or bracket to prevent this. But .... rebuilder could have left this out or ... ?

My second thought is oil aeriation, as already mentioned.