Are you running the stock end seals on your intake? If your block or heads have been decked/surfaced the cork or rubber end seals can lift the intake off the gaskets at the intake ports, especially on the bottom(lifter valley) side. Then the intake runners can suck
oil in right there and it will mysteriously disappear from the crankcase without leaking on the ground. If your valve guide seals are shot or your rings are used up, it'll burn
oil also. Discoloration in the intake runners at the heads is telltale of
oil entering. Sure you aren't leaking?