It's 'steel cut' oats that you shouldn't use for brewing (but it is the #1 best type for breakfast!)
Do you mean unless you do a cereal mash, or is there another reason to not use them?
The would need to be cooked before mashing - a cereal mash would work. Or just boil them according to directions and let them cool. They take about 30 minutes to cook.
I always use quaker oats. I use old-fashioned because that's what I buy for breakfast. But quick or instant would work fine too. The biggest difference in the cooked product is texture (smaller oat pieces), but texture doesn't matter for beer. You're leaving the oat pieces behind in the mash. In theory, instant would be the easiest to convert and extract sugars from.