Re: MC Oil Pressure?
The gauges will become erratic / problematic over time (and 20 years is a longe time, especially in the marine enviroment). I replaced all my 20 year old gauges a couple of years ago with standard off the shelf ones, after having problems similar to yours. It was a "plug and play" operation, as the existing senders were reused. The Mercruiser senders / guages are basically industry standard, as far as the specs and values go.. Unless, of course, they are some over the top / specialized / very expensive ones, which one would doubt. On your oil pressure sender, I would replace it, as it sounds like there is "accuracy creep" in it as it heats up, a common problem. They are a standard sender that is pretty easy and cheap to replace.
You could install a s/s type line from the sender location on the block and mount your senders at the end of that in some accessible location to ease accessing the senders at a later date, if need be. Hot rodders have been doing that for years and would work just fine on an I/O.... JEGS and Summit Racing have huge selections of these. An AN3 selection would be perfect. I would install the S/S ones and not use the plastic lines anywhere, since they can be somewhat fragile / brittle if exposed to heat over time or mishandled in any way.
30psi into one end of the line equals 30 psi out the other end, even if there is air in there, since there is zero flow through the line. The air bubbles (there will always be some) would also act as a slight damper to smooth out any pulsations in pressure, if they did occur.
