Oils like the High Mileage Castrol and Mobil 1 (to a lesser extent) prevent burnoff because they swell the rings.

They also do a good job at cleaning out carbon deposits through their extra detergents which is a mixed blessing. Higher mileage engines tend to have a nice layer of carbon on the piston top which actually bumps compression. With that gone though, you have a much better surface for the fuel to atomize.