I feel like I'm missing something obvious. I've seen several posts where folks (particularly Ian) note that ring, valve, etc. problems can pressurize the crankcase, leading to excess oil in the case and possible wet sumping (most of these discussion are from posts about wet sumping and oil pump function). Here's my question - wouldn't crankcase pressure actually reduce oil flow into the case? It seems like crankcase vacuum would result in excess oil into the case, but positive pressure would not? What am I misunderstanding about the oil flow and airflow in the M20 engine?
The reason I'm asking is I just started having excessive breather oil losses. I checked the breather valve - fine. Anti-drain ball valves are both good. I pulled the oil pump (which was stiff), lapped and reinstalled. I have excellent oil flow, but it never goes dry and sputters/bubbles. So I seem to have excess oil in the case. I started researching causes on the forum, and I'm confused about why positive crank pressure leads to excess oil in the case/sump. It seems like the opposite would happen? 
Craig
Hi Craig...Excessive amounts of oil in the crankcase can only really be caused by three things...
The oil draining through the pump and anti drain valves when the bike is left standing.
The ball in the anti drain valve located in the oil pump base plate being stuck on its seat...This would prevent oil being picked up by the return side of the pump.
A badly worn pump which is not returning oil sufficiently fast...
The latter two reasons would be unusual...
The return side of the pump has a greater capacity than the feed side and this ensures the crankcase is 'scavenged' when the engine is running...
If the oil is draining into the cases when the bike is 'idle', when it is first started up there will be too much oil in the crankcase and some of this will be ejected through the crankcase breather valve...
The crankcase breather itself allows pressure in the crankcase caused by the downward movement of the piston to be evacuated from the crankcase...
On the downward stroke the flap valve in the breather is blown off its seat and the air is expelled. As the piston rises the flap valve is sucked back onto its seat, sealing the crankcase and restricting the amount of air being sucked back in...
The valve itself has no effect on the amount of oil in the crankcases...
However, there is always an amount of residual oil in the crankcase, even when the pump is working perfectly....
If the crankcase is then subjected to additional internal pressure by a worn cylinder for example (lost combustion pressure) this can over pressurise the crankcase, forcing residual oil out of the breather when it opens...
The problem then is not excessive oil in the cases but excessive pressure...Ian
email (option): ian@wright52.plus.com
Thank you both. You are correct that I misinterpreted the previous posts to mean excess pressure equals excess oil. I now understand excess pressure just means excess pressure! 
The odd thing with my situation is that I had no sign of excess oil after standing and idling for several minutes. But after about a 5 mile run, I was idling in my driveway and noticed a 3oz pool of oil under the breather. I had been meaning to check the pump condition anyway, so I pulled the pump (and noted another maybe 6-8oz of oil still in the sump!). The pump was also very stiff, so I dismantled, cleaned, lapped, reassembled, tested turning resistance before and after installation, now all seems good. BUT, despite very good return oil flow, I'm not seeing any indication that the sump is being fully scavenged. But since Ian notes there is obviously always some residual oil, maybe it is getting nearly scavenged. I'll take it for a few more drives and see how it goes. I obviously can only observe oil flow when standing still. I just wanted to see if there was any reason I should suspect valve or ring problems, but Ian has clearly outlined that there are only a few possible sources of excess oil, which is very helpful!
p.s., when I pulled the pump and had the bike standing with the sump plate off for several days, there was no oil loss, so I feel even more confident my anti-drain valve is working as intended. Maybe this was just an odd one-off situation with some sort of blockage in the return or anti-drain passageways.
Thanks again for the tips. At least my pump is now turning well and working as intended!
I would recommend that anyone buying an M20 or any BSA that uses this type of pump should drop the oil pump out of it sooner rather than later...
The Mazak pump bodies more often than not need 'lapping in' to produce a freely turning pump...
Whilst the engine has the power to drive the pump even when it is tight, this will result in a worn out oil pump drive shaft worm far sooner than would normally be the case...and also wear of the corresponding crankshaft pinion worm...
I have around 25 oil pumps...I would say only 5 or 6 do not show to a degree signs of age deterioration of the cast components...and some are beyond serviceable use...Most are tight as they were when they were removed from donor engines...Ian
email (option): ian@wright52.plus.com
Yup. Mine was tight and I'd ignored it for some time. Glad I bothered to pull it out and do some lapping. It's really a very easy job.
Somewhat off-topic - It might be interesting if the forum collectively built a list of important issues (such as oil pump servicing) to keep on the tech info page. Something like a top 10 (or 50! ) list of things any newer owner should check to avoid risk of more serious damage. The maintenance manual and 1961 General Upkeep document are good places to start, but perhaps something based on the collective knowledge of the experts on the forum that just lists important things that might not be as obvious to the new owner. With so much uncertain history on these bikes, it's hard to know where the highest risk or most significant trouble spots could be... especially the areas that don't make them selves as obvious until something has failed. Just a thought...
Hi Craig...That's a good idea..
The oil pump, for example, doesn't appear as an issue in BSA literature because it takes a few decades for the problem to appear...
There are a whole raft of things worth checking on a bike that has been laid up for a long period...Ian
email (option): ian@wright52.plus.com