I really wanted to say that you can use either rolled oats or instant oats, but I'd be lying to you. You will get the best results using rolled oats. That is because instant oats are not only thinner, but some brands also pre-process them, so they cook much faster. In my two experiments, this created a gummier texture. If you don't have rolled oats, you can still make this recipe though. I recommend increasing the amount of flour and nuts to compensate—an extra 1/4 cup of flour and 1/4 of chopped nuts.