[..P]

The question in my mind is, "is the low idle causing the problem of low oil pressure"......or........."is the low oil pressure causing the low idle".

If the the motor heats up, clearances can, indeed, increase and cause rough idling, in which case, if it idles down far enough it can cause low oil pressure and perhaps some clunking.

Your car is a CIS (continual injection spray) type, I believe, so it's different than my 83 with L-jetronic, but the vac still is important, and there are lots and lots of old brittle vac lines under the hood that have been cooked for years. When I replaced spark plugs a while back, I touched some of them and they broke upon contact.

Probably the best thing you can do is to take it in to a shop familiar with the 928, and let them use the diagnostic tools available to find out what the issue is.

My car is set to idle just below 1000 on the tach, maybe around 800 or so which seems a bit high but it runs and drives well at that point.

Here is some moral support for you, buddy, all the best. I hope its a simple fix for you.

Regards, P