Oil Pressure problems.

It's most likely your oil pressure sending unit.You see,the way these senders work isn't like the ones in 80's and early 90's cars.These are dummy senders.Instead of changing the ammount of current to the gauge due to different pressure variations,it's either on...or it's off.0 - 7 PSI and the gauge reads 0, 7 PSI and above and the gauge reads at around half.The sender is just a pressure switch. ( Very dumb ).

Your sender is located in between the water pump and the power steering pump.You'll see a black bracket coming off the side of your PS pump,now look way behind that bracket near the engine block.See that small round sensor with the black plug attched,that's your sender.

On my 96,that black bracket isn't there.But I still had to pull the belt off,remove 2 bolts and one nut off the main ps/alt bracket just so I could shift the whole thing to the side to allow access to the sensor with a socket.Yep..PITA! .So it looks like you'll have to remove the black one also.As for the sender,mine took a 27mm socket.Not sure if it changed.

It's definitely the sender. If it was your oil pressure, you would have already smoked the engine.

It's not low oil pressure after a cold start. If anything, the oil pressure would be higher than normal, since the oil is thicker. For some reason, the switch isn't throwing until it warms up, so replacing it should do the trick!

Being a quart low shouldn't cause this problem. From the min line to the max line on the dipstick is one quart, so being a quart low still puts you in the green. You would have to be well below that for the oil pump to start sucking air.

How long has it been since the oil has been changed? If it's full of sludge, that's the only reason I can think of why it would not show pressure until it has warmed up.

Replace the sender first and see if that fixes your problem.Kuddos to what Skymarshal said.1 Qt wont affect the pressure.You'd have to be below 3 qts and on a hill at the same time to cause oil starvation.

This next bit is somewhat long.It may or may not have anything to do with your oil issue.Worth a mention tho.

A few years ago,my oil pressure gauge started acting all wacky.It would read just slightly past the R in "NORMAL".Slowly over the months it started reading lower and lower on the gauge towards the N.Then it would work like a light with a bad connection.Flick up and down..kind of just jiggle around the gauge..start reading near the N..then surge up near the R.So I replaced the sender.That needle surging stopped.But it still would read at the lower end of the gauge.After 8 months or so,the new sender started leaking oil everywhere.So,I replaced it again.Gauge still reads low ...and then sometimes it will move up a little and stay there for a week.

6 months ago I gutted a wrecked 95 GT for interior parts.I even took the GT's gauge cluster.Mainly I took that because I wanted the 150 speedo and was hoping I could have a working odometer. The Odo. was a no go.I swapped the gauges into the old PCB (GT's PCB wont work).Everything works basically,BUT , even the GT's oil pressure gauge reads abnormally like my old one did.This is obviously an electrical issue.Not with the gauges or the sender.Maybe this is common on stangs.

MustangLX-5.0, that's an interesting problem, and it reinforces my suspicion that Mustangs have conventional linear oil pressure gauges- not gauges controlled by a pressure switch.

I know for a fact that late model V6 F-150s have the pressure switch instead of a variable pressure sender, but even though their engines are the same general family as ours I don't know if that means we have a switch too.

Auto manufacturers go with pressure switches not to save money, but in cases of engines that have a lot of natural oil pressure variation. I used to drive a Probe GT, and those 2.5L engines were notorious for having idle oil pressure that was at the bottom (or just below) the "Normal" range. All the other owners on the Probe board I was on saw the same issue. That was the cause of a lot of warranty claims and dealer troubleshooting, so I'm sure Ford realized they could avoid all that with a simple switch that only signaled a problem if the pressure was actually below the minimum needed. They kept the gauge since it looks more "sporty" than a simple idiot light.

The lower oil pressure reading could be just lower pump efficiency due to wear and looser clearances (assuming that we have a true pressure gauge). Might be interesting to put an ohmmeter on that sender and see if it's true pressure or just a switch!

The below post is my test results on the sender and guage.It's rather long so...take note.

It appears to be switchable on/off.Engine off/key on,out of range or infinite.Engine running,it read at 0 ohms.But heres an odd one.The meter has 2 separtate scales to measure values.The main numeric reading and then a long scale bar at the bottom.The main reading never flinched from 0 ohms.But the scale at the bottom kept fluctuating back and forth between 2 and 6 ohms.I'm guessing this scale is to take the place of an analog readout.The measurements seem to be too fast to read on the main display,but 0 ohms must be what it's average value is.

I probed the sender wire and got 4.7 volts Key on/Engine off.With Engine on I got 5.8 volts.I couldn't find the base voltage for this terminal in the Haynes manual anywhere.Had everything else but that.It's probably 4 - 6 volts.

Looking closely at the gauge,besides the fluctuations I've noted,I also noticed that the needle was twitching.I mean you have to look real close to see it,but it was twitching in a steady pattern.The needle travel of these "twitches" only covered about 1/6 of say...the letter R in "NORMAL".The rate at which it twitched was close to but not always consistant with the idle pulses (or the lope you hear in the engine).

I probably have two problems...possibly.The first,electrical gremlins (eg.. one or more bad grounds somewhere) and the second,the low RPM oil pressure is low enough to just barely cause the sender to constanantly trigger on and off rapidly (eg..between 6 - 7.5 psi) thus showing 0 ohms on the meter yet the scale on the bottom read steady fluctuations between 2 - 5 ohms,allmost at the same rate the gauge needle was "twitching".

If that's not it then the hell if I'd know what was going on with it.

Well, I guess we have the switch! Thanks for testing it out.

I'm not an engineer, but I'd bet that the low voltage is done on purpose to avoid any arcing in the switch that would wear it out prematurely.
I'd also bet that the needle twitching is just due to voltage variations from the alternator as the engine lopes.

The oil pressure gauge is probably "weighted" like the fuel gauge, so a varying input won't appear on the gauge. It's the same design that prevents the gas needle from going back and forth as the fuel sloshes around. If the oil pressure switch is bumping back and forth between off and on, that might explain some of the strange needle behavior!

My F150, 4.2 v6 is doing the same thing. Did the sending unit fix the problem? Just curious.

I have no idea if it fixed the problem on Leanz stang or not.But since rreynold6 updated this post,I might as well add.

I have been noticing the past few weeks that the current position where the gauge needle sits has been gradually sinking.It now goes to the first bold white line before the letter "N" in "NORMAL".Yesterday it moved to between the red line and the bold white line.Today it's back to just before the "N".This was really starting to bug me.Popped the hood and took my meter (It's a MAC Tools EM720 just for ref purposes),and used it to jump the sender wire directly to ground.What I did was disconnect the wire and run it through the ammeter then jump directly to ground.Looking at the gauge,it showed the needle sitting flush on the "M" in "NORMAL".So,it's NOT a gauge problem.Leaving the sender wire connected to the ammeter,I then connected the ground side to the sender.Gauge was sitting on first white line.The current (I) readings on both tests were --- Sender wire direct to ground = 115 mA , Sender wire to Sender = 39mA.I pulled the oil cap and the rockers were flicking a steady flow of oil.So it can't be a pressure issue.Either the sender tube is plugged,the sender itself is faulty,or there is a bad sender to ground connection in the threads.Seems odd but it may be some RTV on the threads causing bad contact.Don't know,but I highly doubt that's what it is.As this problem started many years ago and only has become worse over time.The stock sender,and 2 replaced units making a grand total of 3.....I doubt its a contact issue.