Is it okay to boil milk (making steel cut oats)
Yes, it's fine to boil milk - or at least to simmer it. Milk gets really foamy when it boils, and tends to boil over, so you probably don't want to actually boil it, but rather quickly turn the heat down when it shows signs of boiling, and let it simmer. It does develop a film on top sometimes, but nothing that can't just be stirred in and dissolved, and you should be stirring your oats reasonably frequently anyway so they don't stick to the bottom.