I could live with a low pressure warning light instead of a oil pressure gauge, but I could not stand the idea that something in my beautiful Miata would be an outright fake. So I decided to follow the instructions of David Gould , posted on miata.net, and replace the fake gauge by a pre-1994 real one. I paid about $150 for the parts, new.
Any responsibility in trying this is yours. You are dealing with oil that may burn, cars that may fall on top of you, airbags that may explode, and battery fumes that may explode, to mention just a few of the hazards. Read and understand all information before you start. This includes the instructions below, the instructions that came with your jack, chocks, and stands, and the warnings on batteries and airbags in your workshop manual. Also reread your car's instruction booklet on how to replace tires and the dangers of jacking.
In any case, the entire thing is most recommended for intelligent octopi owning Miata. While an octopus may feel the Miata oil filter is easy to reach, (even with half a Sebring supercharger above it, as in my case), this octopus may still complain about reaching the sender, which is located near the oil filter, but more recessed. I took off the right front tire and the plastic protective bottom engine cover. Not that much trouble, and when you start doing the work, you are glad with any access and visibility you can get.
Leo Change notes that if you use an universal joint for socket, and a long extension, then it is easier to get the sender off.
Needed (at least, this is what I used): Pre-1994 oil sender and gauge; 10 mm wrench to take off the negative battery terminal; 10 mm socket to take off the plastic protective shield below the engine; 12 mm socket to loosen the steering wheel (MM uses a long phillips instead to take of the plastic shroud around the streering wheel); 24 mm (or 22mm?) deep socket to screw the old sender out; 29 mm or 30 mm socket to screw the new sender in (try the actual sender for size!); articulated wrench; 3 inch and 6 inch locking extension pieces; big size torque wrench (for torqueing the wheel nuts); silicone sealant; big and small Phillips screw drivers; flat screw driver; hydraulic jack (maximum height collapsed 5 inch, from Sears), stands and chocks to jack up the car; flashlight; pick up tool to pick up dropped sockets, senders, et cetera; much band aid; radio to drown out the cursing and screaming. Mazda workshop manual recommended!
Preparation: Make sure that the engine is cold. This is *not* the time to start dodging hot spots. Wait a couple of hours if you have just driven. Slightly loosen the right (seen from the driving position) front wheel nuts and jack up the car. The jacking position is in the middle of the cross member (horseshoe shaped steel support member) between the wheels; the precise middle of this jacking point can deform, so make sure that the jack also touches some of the stiffer edges of the center. Place the car on stands: do *not* work on a car only supported by a jack, and especially not now. Take off the right tire and the plastic protective bottom of the engine. (There are two types of screws holding the plastic bottom.)
Locate the sender. It is at about the height of the top of the oil filter, somewhat more toward the rear of the car, and, unfortunately, even more recessed. A wire is sticking out from the end. Take off the wire (I found it easiest to do this below the car by reaching in the blind through the rear part of the right wheel well). Unlike what David found, mine came right off. Screw out the sender. I spend hours attempting to reach the sender from all sorts of positions below and above the car without ever seeming to get a good grip with my 22 mm deep socket. Finally I managed to get both my right hand and the socket together at the sender and figured out that the 22 mm recommended by David Gould was just too small. (David, if you are planning to take any vacation in the vicinity of Tallahassee, FL, it should be safe there in a few weeks ;-) After that, it was relatively easy first to extend my right hand towards the sender from above the car, then to move the 24 mm deep socket and 6 inch locking extension to this hand and finally unscrew the thing using the articulated socket wrench. Note that I have fairly slender hands and arms.
Put a thin layer of silicone seal on the threads of the new sender, except the first few windings (2.5mm). Let dry for one hour. (I did not use the silicone seal the first time, and it leaked a little oil each day: make sure you use the seal. I got mine at Sears.) (MM says to wrap the threads with Teflon tape instead. Wind it in a clockwise direction, as viewed from the nipple's end, and only make two wraps. Make sure that the extra tape will not protrude into the oil passage.)
David lists the size of the socket to put in the new sender at 29 mm, but it is not a precisely machined part. I found that a 30 mm fitted mine better. Try the size that fits best before putting the sensor in. Then I used a 3 inch wobble extension and the articulated socket wrench to screw the sender in. A 6 inch extension is too long to fit, but the 3 is really too short. David notes that the tightening torque for the sender is 13 ft lb, but since you won't get any torque wrench in there, just make it 'not too tight'. Note that if your wrench is about a foot long, you should not put more than about 13 lbf on it. That is not much force: try it out on a scale. But don't let it come off either! Put the wire back on. Put the wheel back on, hand tightening the nuts. Check below the car and lower it, removing chocks and stands. Torque the wheel nuts to 66 to 86 ft lb (see your car's instruction booklet for the correct numbers for your car). You need to retorque these nuts after about 500 miles. Write it in your planner.
Do you know your radio's code number? Disconnect the negative terminal of the battery (I forgot, but you definitely should do that. Then again, MM does not require you to disconnect the battery either.) Careful, batteries contain corrosive acids and produce gases that may explode. Don't drop things on the terminals or create any conducting path from the positive terminal to the car or negative terminal.
Since you need to work around the steering wheel, there is some danger of the steering wheel airbag exploding and injuring or killing you. Since the sensors are at the front of the car, I would assume the greatest danger is in electrical mishaps. Even though you have disconnected the battery, the blasted airbag (hate those things; they are expensive, ugly, and have apparently no clear advantage in preventing injury versus causing it either) has a backup power supply. The Miata Enthusiast Shop Manual seems to suggest to disconnect the orange and blue interlocked clock spring connectors (don't ask) below the steering column, the smaller orange one first. I did not; and MM does not mention it. You make up your own mind about the risks you want to take. In any case, don't put anything on the airbag (it may be thrown back at you) and do not play around at random with ohm or multimeters; they may produce enough current to set off the airbag. And no, do not hit the airbag with a hammer or similar, even if you want to.
Inside the car, take off the two Phillips screws below the instrument glass cover. After that, MM tells you to use a long phillips screwdriver to remove the plastic shroud around the steering wheel. Then you should be able to take off the instrument shroud by grasping it with both hands near the top sides and pulling it hard toward the steering wheel. It should release with a loud "pop". (Instead I took off the iron cover below the steering wheel by taking off the two Phillips screws and rotating it out. Careful: an exploding steering wheel air bag can kill you. Then I took off the two 12 mm bolts on the flange that keeps the steering wheel assembly up. The steering wheel assembly drops down and you can now work off the instrument hood. I would assume MM's approach is better, although I did not try it.)
Next take off the glass piece by releasing the catches and any screws. It is still held on by two wires. Make a note of the position of the wires and take them off. (I did not, so I had to guess where to put them back.) Careful, do not damage your gauges! Now unscrew the four Phillips screws keeping the instrument cluster attached. Take off the two cable plugs behind the cluster, by depressing the catches. Take off the round plug. Now you can carefully rotate the cluster far enough forward to reach the back. Take off the three screws that hold the fake oil gauge in place. (Look at the replacement gauge for their position.) Put the real gauge in. Reassemble in inverse order. (The torque on the flange bolts is 14 ft lbf, if you followed my approach.) Reconnect the battery.
Run the engine and check for leaks. Check operation of the oil pressure gauge. Check for tools whenever you close the hood.