[Christ]

I think my vacuum number was incorrect. At 15", it becomes a requirement to reverse the direction of the seals to hold the vacuum, since at that level, the oil simply can't get out.

I actually believe that you want to keep the number under 3 inches, preferably close to equilibrium.

The point of the vacuum system is to provide a higher pressure differential across the face of the piston during combustion, and to aide with ring sealing, preventing cross contamination of the combustion chamber and oil. There is also an apparent benefit to the valve seals.