See My edits. New oil and filter good idea. 30 wt is good. Nothing wrong with the motor just the monitoring system. I don't know if Mystery type oil additives are good or not. If they are, throw some in if they actually clean passages/holes. Who knows?

It may be possible to find another pipe thread port on the motor suitable for a direct/no tubing gauge to use as a reference to the other gauge. That is what I would do and have done.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn