An airplane in Canada is legally required to be equipped with a working oil pressure gauge for each engine. When you start a piston engine, your eyes go to the oil pressure gauge. You're waiting for the pressure to come up. Given that it's the first thing you do on starting an engine, it's one of the first things I learned as a student. (Right after, "in order to start an engine, you have to turn the key to the start position and hold it there until the engine has actually started," which was explained to me immediately after I did it the wrong way. Yeah, I was pretty dense). If the pressure doesn't come up, you're supposed to shut down the engine and not do anything else.
I'm not sure I've ever followed that instruction. I am starting an engine that I have just verified has oil in the crankcase and does not have a huge puddle of oil on the ground underneath. And I know how hot or cold that oil is, from 'so hot that I had to use a rag to handle the dipstick, and couldn't really see a level anyway', to 'I just spent twenty minutes deicing and I can't feel my toes or fingertips'. If it's the latter, yeah, I know that oil isn't going to come dancing through the tiny line to the pressure gauge in any big hurry. I will give the oil a while to warm up, and thus the oil pressure more time to increase.
In retrospect, I wonder if disciplining student eyes to go straight to the oil pressure gauge was a way of training us to be vigilant for a hot start when we move on to turbine engines.
High oil pressure can be a sign of a blockage, or you're running summer weight oil too far north, so instead of distributed cooling slipperiness, the oil is a viscous glob having trouble getting forced through some part of the system. Yeah, oil that is too thick can cause high or low oil pressure indication, because unless you're short on oil, both high and low oil pressure indications are caused by poor oil circulation. The pressure is high one place and low another place. Or it's the gauge. Low or high oil pressure, it's usually the gauge.
Even when its probably not the gauge, so long as there isn't oil splattered all over the airplane, and the engine didn't quit, the pilot rolls up to maintenance and says, "I have a low oil pressure indication, but it's probably just the gauge." You never want to give maintenance the idea that you are terrified of the airplane they gave you. I'm not sure why we do this. Perhaps I should try the opposite strategy sometimes and insist that ... oh wait I did that last week with a stuck propeller cable that turned out to be ... maybe I'll admit that one another time. While instantly diagnosable by someone leaning into the cockpit from the right-side door, it really wasn't visible to someone strapped into the left seat. Anyway, under-reporting of the severity of our concerns seems to be a pilot thing. Maybe because maintenance likes to show us to be wrong, so if we show little concern, they will work hard to find some way it could have killed us. Which is what we want them to do.
So my story is, I report unequal oil pressure on several consecutive flights in this airplane. It's in the green on both engines, I concede, but used to be equal, I think. I didn't remember there being a split, and I would. A pilot scans the engine instruments far more times than she knows, because we only notice we're doing it when something is wrong, and the two needles for the same thing on the two engines not being in the same position on their adjacent dials comes up as wrong, even when they are both in the green. The oil pressure indication on the right engine is inching up slowly. If it were a single engine airplane I probably would not notice. That's something I say a lot, but I keep reporting these things, because sometimes it matters. It's remarkable what a tiny thing can be a symptom of.
Eventually the needle reaches the yellow and they take it apart and find out what was wrong with it. An oil pressure gauge works the only way one logically could but I was still kind of flabbergasted when I realized that a little pressure line comes off the oil system, ducting a bit of the engine-pressure oil right to the instrument in my dashboard. Its internals consist partly of a ball bearing and a spring, and the pressure of the oil on the ball bearing compresses the spring, driving the needle. The spring was worn out.