IF there is a gain, its so minimal its not worth the effort. there is no way to get the engine into a large enough of a vacuum to reduce drag where it would make a difference.
look at it this way, how much do, pistons, rods, crankshaft, camshafts, accessory drives, flywheel, clutch, transmission, driveshaft, rear end, axles brakes wheels and tires weigh? a lot right? the air in your engine, doesnt reduce ANY reciprocating mass therefore would do no good in removing it because you still have to turn all of that ****.
and you could still have oil leaks. oil leaks arent there because of pressure in the engine. they are a failed seal.