Proper Miata oil pressures?

My Google fail and GRM search fail both leave me to ask: what is the proper oil pressure for a 1.8L Miata engine?

I have a 1994 with 150K miles. In the summer (100 degrees F, altitude 4300 ft), after 30 minutes of highway driving and with 5W30 oil the engine produces about 32 PSI oil pressure at 3200 RPM. With 20W50 it produces 48 PSI at the same speed. With the 20W50 the pressure tops out at about 51 PSI - looks like the pressure relief valve is opening. With 5W30, pressure never tops 37 PSI.

Idle pressure is usually around 15-20 PSI when warm.

I think whatever the gauge displays is proper.

That sort of mileage is usually around the time i start to run an xw40 oil.

Gut feeling tells me that on the highway, you'd want to see about 40psi, maybe a touch higher. But... that's just what i should be seeing on my Mazdas, the Miata may be different.

I think mine reads ~40-45 PSI at highway speeds. Its a 94 with 88K and I use M1 0w40.

Having to use heavier oil to increase pressure is generally not a good sign.

My Zetec manual says 54 psi hot a 4000 rpm. On 5w-20. I know, it is not a Miata.

I have an aftermarket gauge with a switched warning light in my 51k mile '96, that comes on at 20 psi. When the oil is at full temp, the light comes on at idle with 5W30.

Doesn't sound too bad to me. Run 10w40 if you want, but those numbers aren't completely out to lunch. Especially not for 150k miles.

Rule of thumb for those with older, tired engines: 10W40 minimum, and 10psi per 1k rpm. For those of us with older British cars, they were designed for 20W50. PO of my Spitfire put in 10W30. "I don't hear that rod knock"

the numbers in the original post are not far off from what my '94 does, with 98k, on the unknown oil that was in it when i bought it. i show maybe 2 or 3 psi higher. then again, that's at full water temp, but just some around town driving. so maybe my oil temp isn't up there yet. i haven't had it out on the interstate.

just bought some 0w-40 to put in there.

One note about those 1990-94 oil pressure gauges - corrosion on the spade connector on the sender will give you a lower reading. A bit of emery paper will clear that up. I've also seen the senders clog up inside, which will also drop the reading.

Ah, my oil pressure numbers come from an Autometer gauge added to the IP, with the sender screwed into the oem sender port. I've calibrated this set-up with a known good mechanical pressure gauge.

For your numbers, I didn't suspect the gauge. They sound right for your engine. But I've seen a 1990 showing 5 psi at idle on the factory gauge when it was really making 30. That was more of a public service announcement

Keith wrote: For your numbers, I didn't suspect the gauge. They sound right for your engine. But I've seen a 1990 showing 5 psi at idle on the factory gauge when it was really making 30. That was more of a public service announcement

Got it. Thanks to all for the helpful posts! I feel a bit better about the old engine now.

0w-40 Mobil1 bumped it 5-10 psi !?