My '92 Miata (stock 1.6, 88k miles) has gradually had lower and lower oil pressure at idle. It's now to the point that it's got to be less than 10lbs at a warm idle. Going down the road at 3-4k RPM, it holds more like 35-40 lbs (which still seems low to me). This morning on the way to work, I noticed a faint rocks-in-a-coffee can noise coming from the engine that went away as soon as it was revved above idle, and returned as soon as it idled back down.
I'm running Mobil1 Extended Performance (or whatever the 15k change interval Mobil1 stuff is called) in 5W30 with a WIX filter, it's been about 2.5k miles since I changed it. Oil is up to level and looks clean. This is my first go-round with the Mobil1, prior to that I've used Castrol GTX and a WIX filter going back to when I bought the car with 35k.
Am I looking at a rebuild soon?
I'd try switching back to the castrol dyno oil before freaking out.
Sorry, meant to say that this has been gradually worsening for a long time. It was still low on the dino juice, and has been getting worse over the last 10-15k miles. The Mobil1 did make my HLA stop chattering, though.
Oil pressure should only be checked at normal RPM.
As long as it is good then the idle will be OK.
28 to 43 at 1000 rpm.
35 to 65 at 2000 rpm
Doesn't seem that at only 88K miles the oil pressure would be a problem.
Check it with an independant gauge.
Installed gauges are often in accurate.
First thing I'd do is clean the connector at the oil pressure sender with some electrical contact cleaner and maybe a little steel wool. This might magically increase your oil pressure 
.
Either way it seems awfully low for an engine with that low a mileage. Do you know anything about the car's history prior to you getting it?
Bought it in 2005 with 31k miles, it was immaculate and came with service records. I've let a couple of oil changes go beyond 3k, but no more than 5k.
I will try to get a different gauge on there. Think I've got an old aftermarket OP gauge somewhere.
The gauge won't indicate more than about 15-20 at 1000 rpm, and probably 25-30 at 2000. Whether that's accurate, though, is a good question.
If it's really that low, I would be looking at the relief valve.
Most overlooked part on an engine. 
