Q: A friend drove my old pickup yesterday and commented that my oil pressure was lower than he was used to seeing. Frankly I hadn’t paid much attention to this because it was never in the red. The gauge shows about 20 when I’m driving and about 10 when I’m idling. There’s plenty of oil in it. Should I be worried? It’s a 1982 GMC with 156k miles and runs great.

A: I agree the oil readings are of concern. I’d rather see about 15-20 pounds of oil pressure at idle and 30-50 pounds at higher engine speed. Either your engine has lower than desired pressure, or the gauge is incorrectly reporting it.

Let’s start with the oil. Is it fairly clean and of reasonable viscosity (thickness)? If it’s contaminated, particularly with gasoline, or perhaps 5W-20 oil was used instead of 10W-30, this could be part of what you’re seeing. Older vehicles such as yours using a carburetor typically employ a mechanical fuel pump, attached to the engine. Should the fuel pump’s diaphragm start to leak, gasoline can enter the engine crankcase, diluting the oil. If the oil has a strong gasoline odor and seems oddly thin, renew the fuel pump.

I’d continue by renewing the oil-gas evident or not. If the oil pressure doesn’t improve, try connecting a mechanical pressure gauge to the engine. This is done by removing the engine’s oil pressure sending unit (top of engine at rear) and temporarily installing a test gauge. This tool can be inexpensively purchased at an auto parts store or built from hardware store parts. If the test gauge indicates normal oil pressure, the incorrect reading may be caused by a faulty gauge sending unit (most likely) or a problem in the gauge cluster. A faulty sending unit would be a sweet and inexpensive fix!

If the test gauge reports low pressure, the most likely causes are worn engine bearings, a worn oil pump, or sludge restricting the oil pump pickup screen. Worn crankshaft bearings can be noisy (a deep thumping or castanet sound), while worn camshaft bearings are typically unnoticeable. During the oil change, does the oil drip freely or are there visible blobs? You can also try swabbing various areas of the oil pan (via the drain plug hole) with a self-fashioned probe. If it comes out with blobby deposits, or sludge is noted at the oil filler cap area, this may be an avenue to pursue. With already low oil pressure, I’m leery of adding a flushing chemical other than perhaps a half-quart of automatic transmission fluid (a great solvent), and changing the oil again the following week.

If a large quantity of sludge is observed, and/or fails to clean up, I’d take action soon by removing the oil pan, cleaning everything in sight, and while you’re in there, renewing the oil pump with a high volume replacement. If sludge isn’t evident, perhaps you might live with the low oil pressure, band-aid it a bit with 20W-50 oil, and keep on driving.

With a little luck the engine may last another 50,000 miles. When the time comes, if you wish to renew the engine, GM markets a crate (new) engine (part 10067353) for about $1,850, which is a simple and comparably affordable option.