Still curious how negative pressure would affect oil as far as leaking out of the block.
Does it affect it at all?
Since I've never run a pump I have no idea.
I have multiple engines with vacuum pumps/vacuum systems on them. One would think that if you are pulling a vacuum in the crankcase that it would not be possible to leak oil out but that is not true. I had a BBC that leaked oil on the right side, it took forever to find, there was a pin-hole in one of the fabricated valve cover welds. The engine had 10-12" of vacuum (wet sump) yet still leaked out the pin hole. I also had a front seal leak on a circle track SBC even with 10" vacuum.
The interesting thing is that if it can leak oil out it can definitely leak air in so by fixing the leaks you should see more vacuum. The other thing to consider is that if it can leak air in it can also leak dirt in. This isn't a big concern on a drag race car but it can become a big concern on a car that is driven in dusty conditions. It doesn't take much dirt to ruin bearings.
The biggest advantage to running negative pressure in the crankcase is that it allows us to run ultra-low tension oil rings and still maintain ring seal. This reduces parasitic losses. You can't run 5-8lb tension oil rings (at least for very long) without having vacuum in the crankcase as they won't seal. It is absolutely imperative that you keep oil out of the combustion chamber so you can see how important vacuum becomes.
I did a test once with a methanol motor. We were making dyno pulls with VP M1 and no top-end lube. We added the recommended 1oz of top-end lube per gallon of gas (or maybe it was 1oz per 5 gallons I don't remember now) and the engine immediately LOST 10 HP!!!! We removed the fuel with the oil in it and the power came right back. We then used VP's top-end lube which is lead based, not oil based, and power remained the same.
This should show you how important oil control is. If you are pulling spark plugs on your engine and the threads have oil on them you still have some work to do on the oil control. Adding a vacuum system will help greatly with this, as will the right ring package, the right cylinder wall finish, round cylinder walls, etc. Another thing to consider is bay to bay breathing. If you have an engine with poor bay to bay (meaning air can not move around freely in the crankcase) this can hurt ring seal as well. Vacuum systems will help with this issue some as well but I personally don't think you can have too many crankcase vents (internal of course) to connect the heads, valley, and lower crankcase together. The easier it is for air to move around in the crankcase the easier it is for the rings to remain sealed.
These are my thoughts on the issues anyway.