Many cars only come with an "oil light" used to indicate low oil pressure. However, the problem with an oil light is that it usually does not come on until you are dangerously low on oil--not good. Cars that have an actual gauge will usually have an electric gauge, which uses an electric sending unit in the engine to send a signal to the gauge through a wire. While the electric gauge has a needle that moves, it basically only tells you that there is oil pressure in the engine. The best type of gauge is the mechanical oil pressure gauge, which physically reads oil pressure directly from the engine through a small plastic or copper tube. This is the most accurate type of oil pressure gauge; with a mechanical gauge, if your oil gets a quart low you will see it on the gauge.
Locate the oil pressure sending unit on the engine. The location varies based on the vehicle. Look closely at your engine for a round, copper coloured object about 2 inches long and as big around as a quarter. It will have a wire connected to the end of it. Disconnect the wire from the sender. Verify that you have found the oil pressure sending unit by briefly starting your engine. If you have a light, it will stay on; if you have a gauge, it will not move. Use a zip-tie to secure the wire to a safe place away from the engine, such as the wiring harness. Remove the sending unit with a wrench.
Locate the double male-end compression fitting in your new gauge kit. It has threads on both ends, and a six-sided section in the middle so you can turn and hold it with a wrench. Look closely at this part. You will see that the threads on one end taper slightly. This is called a "pipe fitting." The other end is where you will hook your oil pressure line. Check to see if the threads on the pipe fitting end match the sending unit you just removed. If they do, wrap two turns of Teflon tape around the threads in the opposite direction of the threads, and tighten the fitting in the engine block where you removed the sending unit. If the threads do not match, find one of the adaptors in the gauge kit that matches the old sending unit, wrap it with Teflon tape and install it in the engine block. Wrap the compression fitting and tighten it into the adaptor.
Slide a compression nut onto the plastic tubing. It looks like a little six-sided cap with a hole in the centre of it. Slide a ferrule--which looks like a little brass bead-- onto the tubing. Leave about 1/4 inch of tubing sticking out of the ferrule. Place the tubing against the male end of the compression fitting in the engine block. While maintaining pressure on the tubing, push the nut onto the fitting and tighten with a wrench.
Route the tubing carefully into the passenger compartment by locating a place on the firewall where wiring runs through it. The firewall is the rear part of your engine compartment, to which the brake master cylinder is attached; you may be able to slide the tubing through there. If not, choose a spot on the firewall and drill a hole. If you have to drill a hole, insert a grommet into the hole. A grommet is a rubber ring with a groove around the outside edge that fits over the metal. This will protect the tubing.
Choose a place to mount the gauge bezel (holder). It attaches with two screws. The usual spot is somewhere along the bottom edge of the dashboard. Use self tapping screws to secure the bezel to the dash.
Route the oil tube to the gauge bezel. Cut off excess tube with a razor blade. Slide a compression nut over the end of the tubing, followed by a ferrule. Push the tube against the compression fitting on the back of the gauge. Thread on the nut and ferrule, and tighten with a wrench. Hold the compression fitting on the gauge with a wrench, so it does not spin in the gauge.
Remove the two nuts and washers on the bracket that is attached to the back of the gauge. Mount the gauge in the bezel and replace the bracket, washers and nuts on the gauge to hold it against the bezel.
Start the engine and check for leaks.
You can use copper tubing instead of the plastic tubing that comes with the kit. Copper is stronger, does not melt and is abrasion resistant.
Avoid hot engine parts. Allow the engine to cool before working on it.