Every microwave I’ve ever looked at from a distance, or actually used, has the ability to set the power level, typically in 10% increments.
The typical workflow is: power button > number button 1-9
My daily driver is a bit more fancy, and has a single button to cycle through the available power settings.
For your milk oats I’d suggest a power setting around 30 to 40% and increasing the cooking time by a factor of 2.
Sure, but you want to go as hot as possible to get the oats cooked. Besides, I wanted to be as quick as possible. Like, quick oats, you know? ;-)
The majority of those microwaves pulse the power on and off at intervals, sometimes very long intervals (measured in seconds!).
Good microwaves, with an inverter (see the thread above!) actually adjust the power being output. You can evenly heat a cup of liquid to any desired temp using these microwave ovens. They are well worth the few extra $s (although mine actually cost less than a fancy smart microwave!)