Your oil gauge really shouldn't be moving a whole lot. That's a feature, not a bug. The information you need is too cold, too hot, or just right. Knowing the exact temperature might obfuscate the important data. 99.9999% of the time, your oil temperature is within operating temperatures. You really only need to know when it isn't.

Kind of the same thing for oil pressure, you either have enough, or you don't. There's no need to buy a quart of oil just to throw half of it in the car. That warning comes early, it's "you need to go get oil in the car you're currently driving, make that the next thing you do." Not "you're out of oil pull over."

If you ever see an externally mounted oil reservoir, it will have two lines. One near the top, and one a ways from the bottom for max and minimum fills. If the machine that's connected to has a sensor, it goes off well before that minimum fill line, and that minimum fill line leaves enough oil in the reservoir to lubricate everything it's lubricating multiple times over before it runs dry.