You basically can't get 90% efficiency from burning oil in a power plant. E = 1 - Cold/Hot, where cold ≈ 300 K and hot ≈ 1300 K (gasoline-air fire), giving theoretical maximum efficiency of 77%. In practice much lower, what I've seen is just over 60% even in the best combined cycle power plants.

If you use some really high temperature chemistry, you can get to 5260 K from dicyanoacetylene in oxygen, which is 95%, but not gasoline. And dicyanoacetylene is a bit explody.