Engine trouble- Plummeting oil pressure

It's just physics.... the lower pressure is a sign of more flow.....really. The pressure sensor pickup is located at the start of the oiling chain so does not tell you what you have way down the oiling chain. A thicker oil will show higher pressure at the gauge, but the pressure drops will be higher all the way down the chain, and you might be shocked to see it be lower down at the mains.

As long as the oil circulates constantly to all parts, you are BETTER off with a synthetic. If you ever had oil system issues and found out how a full synthetic helps things survive sooo much better, you'd be a believer.

Now a pick-up issue IS an issue. But I am having trouble imagining a decent pump not picking up a thin synthetic; it ought to do so more easily in fact. So I suspect a worn pump....