Well a new engine, consuming oil, means oil is getting into the combustion chamber which is not good for efficiency and/or minimizing knock. So break the engine in moderately hard.
Changing the tranny fluid on an Auto will have a more pronounce affect on MPG, than doing the same with a Manual..
I agree with Frank, find the problem/s and fix them before bothering with any aero mods.
Also accelerate briskly up to speed, and then hold it steady. An MPGuino will help you with this.
