can't see the grease being the problem, although as the others say, I've always used oil... the grease could carbon up on the piston, but i just don't see it causing your problem.

I once heard of an engines mystery oil burn turning out to be a blown diaphram in the brake booster unit, and it was sucking out brake fluid...but that is obviously not your problem.

I'm assuming you have no blowby out the crankcase vent?(pcv)

well, speaking of PCV.. here is a longshot for ya... MAYBE you do have a crack on the bottom of the intake manifold, and the PCV is pulling down the pressure in the engine, and thus you are not seeing a vacuum leak. Try pulling out the PCV and plugging it off... see if you detect a vacuum leak then.