I've had a bunch of Traxxas engines. Most of the "air leaks" people spoke about was actually just due to improper carb setup. Tons of people are told via instruction manuals that a 1.0mm idle gap is normal and that is the heart of nearly all tuning problems.
If I have an engine that won't tune right, the first thing I check is the fuel system and carburetor. If that checks out, then checking the piston/liner fit is next. A poor fitted piston will cause the engine to not idle right and nearly impossible to tune. A properly setup engine will sometimes weep a little oil from the front bearing, but it shouldn't be much and is normal.