Q: Question About Oil and Filling

Hi, I recently bought a pickup a couple of months ago and have some question about the oil. So I checked the oil, after letting the truck sit for an hour, and it was at the bottom of the operating range. I went out bought some synthetic 5w-30. This leads to my first question. I do not know if the oil in the truck is synthetic, or regular. Does it produce harm if i use synthetic and the oil already in the engine is regular? Anyways, Then i checked the oil again, after letting the truck sit for about half an hour, the oil was a little above the operating range.

Going off of what I saw before, i decided to put about half a quart in to make sure. Went back out a half an hour later, and the oil level was still a little above operating range. Did i do something wrong at all? Will this hurt the engine/ is it safe to drive without causing harm to the engine? And last question, what is the wrong that can happen if the oil is below or over the designated oil level range?

Thanks!

My car has 235000 miles.
My car has an automatic transmission.