Sorry, my manuals are packed away right now and I can't recall the pressure vs. RPM curve, although it is typically spec'd as minimum at idle and a maximum pressure that is controlled by the relief valve in the pump. Hopefully, someone else has their "bentley brick bible" near by and can give you the number.

I have only used the oil pressure switch hole to hook the gauge to, so from personal experience, I can not comment on the 6mm bolt you mention.

Once you do have a gauge hooked up, if the pressure is low, don't panic quite yet, especially if the PO didn't keep up with oil changes. A friend of mine in Ohio has resolved a couple low oil pressure engines by dropping the oil pan (PITA) and cleaning the 99% clogged pickup screen on the end of the oil pump pickup tube.

Please note that I'm assuming that you aren't hearing a bunch of clanking from the engine while at idle or otherwise. If there is a bunch of clanking, it's time for a different conversation!

jorrell
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92 245 250K miles, IPD'd to the hilt, 06 XC70, 00 Eclipse custom Turbo setup...currently taking names and kicking reputations!