nippymog, being a simple chap, I ever need a fuller explanation than most, and thus prefer to see a fuller picture, my addendum was typed in that frame of mind... (-:
Further to that, when one last journey was completed the engine oil would be hot if not very hot and thus more viscous/thin which seems likely the most likely time for maximum flow in the drain down process, slowing as the oil cools, but even old cold and thicker oil could drain back down into the crankcase in time..?
If I remember correctly the scavenge side of the oil pump was of larger capacity than the pressure side, in order to minimise the amount of oil remaining in the crankcase when the engine was running. I guess that being the case(?) that it could also be reasonably efficient in returning crankcase oil level increase as the result of drain down during lay-up..?
It seems that thin synthetic oil may be required relative to the clearances, material specification and hotter running temperatures of modern engines to assist them to comply with emission regulations (?) and on that point in times past crankcase ventilation was open to atmosphere to avoid pressure build up and reduce oil leakage... IF the modern M3W engine like many others has to swallow it`s crankcase "breather" output that seems likely to be via an air/oil separator feeding back into the inlet tract, and if you take that a stage further, starting an engine with a high oil level in the sump due to a period of lay up, then it seems possible that more pressure seems likely to be created by the crank in what is now somewhat restricted air space...? More pressure seems likely to create more stress on oil sealing of the crankcase and perhaps more pressure on the air/oil separator mechanisms operation, which then may have an impact on mass air flow sensing if higher levels of oil mist is present on start up and subsequent running till the normal operational crankcase oil level is restored...?
Further to all that... it seems many modern car engines suffer intake manifold fouling from the time that direct fuel injection (DFI) was taken on board. Apparently even some performance engines can start suffering a drop off in efficiency in as little as circa 30K miles due to this aspect of emission reduction. Additionally the air oil separator (AOS) is a prime suspect when things really foul up. I have heard where the inlet manifolds have to be stripped off and cleaned out, and where the inlet tract valves in place are blasted with a natural media to clean off the gunge that builds up...
If you have a carb and likely minimal separation of air and oil fed to the air filter, then whatever oil enters the inlet tract has the benefit of fuel to wash it into the combustion chamber and burn it away .
Amazing the knock on or domino effect of a little too much oil in a crankcase.... or have I got that all wrong...? More than happy to be further/better educated...
