A 11 lb autiomotive valve can only open and allow unfiltered, uncooled oil to bypass the cooler and filter resulting in engine damage if its original with standard volume oil pump, unless bypass is hung open, i doubt its your problem though. Im of the understanding mercruiser used the stiffer bypass valves but not 100% sure. The lines, cooler and bypass were sized appropriately from factory for stock clearances and standard oil pump and usually work for hundreds of hours. Now, if someone put a high volume oil pump in, then it could be bypassing the cooler and filter, smitty

Gotcha, I'll check with another oil pressure gauge first as I'm lost as to what else could cause the low pressure, aside from what you mentioned.