My experience with air cooled engines was an influence in my comment on heat transfer. Paint will insulate, and you don't want to insulate an air cooled engine. Water cooled engines are likely less affected by this phenomena (if at all), but that question of why would you want to paint your block still stands. Like AZ said, maybe you are talking about the goods up top, which make the pics of my new Roe right on topic. And, on that topic, from my understanding, most quality automotive paints will work. You get what you pay for could be the rule of the day. So buy good paint and you'll be OK?