Yes, engines will operate adequately at some level lower than the minimum, but this should be considered "abnormal" and not done casually. If you are checking oil on a warm engine, you can expect a lower reading, as some of the oil is still up in the engine. Should be half a quart difference or so.
Some engines seem to find their "happy" quantity, ans as you get to know a particular engine, you will get to know it. Both my engines are "happy" at a little less than the maximum. My O-360 is very happy at 6 quarts indicated. If I put in 7, it will reduce itself to 6 shortly, then remain their for some time.
Air cooled engines consume oil at a much greater rate that today's car engines, so don't compare. Oil is a consumable. The Tiger Moth I fly occasionally, is literally "top off the oil, and check the fuel".
If the engine you're dealing with is high time, it'll be burning more oil. That's just the cost of running high time engines. Aside from the cost of the oil, it's not so bad - a constant oil change!
If the engine specifies 6-8, do not operate it at less than 5, but ask the maintainer why it needs so much oil, they should have some explanation. Keep a few quarts of your own, so you don't have to worry about the supply being locked up. Never use non aircraft engine oil in it! Just don't fly.