Cyclone!
I've had that same problem when I used Royal purple Oil Racing#9 weight. It did make 10 more HP on the dyno but I would not recomend it to be used in a pleasure boat!!!The 6th pass I had that problem with it not wanting to build oil pressure When it was up to temp.So I would not recomend a thinner oil!
My problem was that I started to wipe out a couple of main bearings.
Now I'm just an a$$hole !!!!!!!????
So dont listen to me I'm just saying that I had almost that same problem.
???Maybe your pump pick up is cavatiting when you excelerate hard,It might just be in the wrong location and not the pick up or the pump at all! eek! I'll start smokin my crack pipe now eek!
And see if any of my buds have run into that same Problem!!