Date registered: Feb 2008
Vehicle: 2004 W220 (S430) ; 2001 R129 (SL500) Sport
Location: Alabama Gulf Coast--formerly Charlottesville, VA
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I'll take a stab.
In the old days, engines used what was called "splash oiling" where the counterweights on the crankshaft went through the oil in the sump and splashed it all over everything on the bottom end of the engine. Then came oil pumps and pressure lubrication systems. The result is now if you overfill the sump and you start "splash oiling" again, you cause the oil in the sump to aerate and foam, and the oil pump won't work as it should. The ultimate outcome can be engine damage. Modern lubricants are much different than those in "splash oil" systems, and they don't like being splashed around.
Also, if you splash oil up on the cylinder walls, it gets past the rings and into the combustion chambers, screwing up the fuel/air mixture, confusing the EMC, and crossing up the pollution control systems. Again, engine problems may result.
That's about 85% knowledge and 15% wag, but I think that's the heart of it.
By the way, if you would take time to fill out your profile, it would help make the advice you seek on this site more to the point. From your post, I concluded you may be from Europe (you didn't say litre, so I ruled out the British Isles). It would help.
"Don't pick a fight with an old man. If he is too old to fight, he'll just kill you."--John Steinbeck