Why does my oil pressure keep changing?

Oil pressure is directly related to

• the ambient temperature
• the engine temperature
• the amount of oil in the engine
• the engine speed

When changing the oil you often end up with a slightly different amount of oil in the engine then you had previously.  Less oil means less oil pressure - add a little more oil and the pressure will rise.

Also, the oil pump is driven by an accessory belt. Operating the engine at higher speed runs the oil pump faster, raising oil pressure. Consequently, oil pressure is lowest when the car is idling (and hot), and will rise during cruising.

Normal oil pressure on these cars is between the Low and Mid mark when idling, and between the Mid and High marks when cruising. These marks, unfortunately, do not correspond to any known psi rating.

Limited testing by DSM owners with mechanical oil pressure gauges indicate that DSMs tend to have around 70-90 psi of oil pressure while cruising, and only 10-20 psi while idling. Cold starts can generate oil pressures of over 100 psi, a fact verified by the DSM manuals when discussing proper oil filter selection.

Consistently low oil pressure while driving could be an oil leak, or a bad oil pressure sending unit. Be careful.

The Last Word: Some DSMs just show low oil pressure, period. My car has shown low oil pressure for years, but nothing comes of it.

And, despite what some people have told me, oil pressure DOES depend - a little bit, at least - on the amount of oil. I've seen the oil pressure on my car go up by adding a bit more oil. There is a danger of overfilling the oil and causing oil "frothing", but I consider that a low-probability problem, whereas my car (and most DSMs, by now) does definitely leak and/or burn oil. Personally, I'd rather have a bit too much than not enough.

The oil light on the dash may come on under hard braking when your oil is low. Top up immediately.

Finally, the little connector that connects the oil pressure sender to the dash gauge can wear out and/or fall off. The gauge will show zero oil pressure and scare the bejeezus out of you, but the dash oil light doesn't light up. The sender is located between the oil filter and the wheel, under the car.