all good points. however, i can't add my own sugar to non-fat condensed milk in this case. the dulce de leche is made by boiling the unopened can in water for 2 hours. at work, we take a case (6 #10 cans) and boil them in a 40gal steam kettle. at home i am making it with one small 14oz. can. the tricky part is making sure you watch it and not let the water level go below the can.
since the fat will likely ruin head retention anyhow, i am sticking with regular sweetened condensed milk. only thing in the can is milk and sugar. you couldn't do this with unsweetened condensed milk, if there is such a thing. has to have the sugar in there.