Is it OK for oil pressure guage to move around within what is marked as “normal” range - reading lower while idling and reading higher while driving?
Yep. That’s how they work! As the engine speeds up, so do the drive gears in the oil pump causing a higher pressure and higher volume of oil to circulate through the engine. As the oil heats up, you might notice a very slight decrease in oil pressure but most times, it is undetectable. Lowering the engine RPMs lowers the oil pressure. At idle, anything near or above 20 PSI—don’t worry about it. It’s all normal.