Yeah, vehicle and engine manufacturers have no idea what they're doing.
I don't work in engineering, but I can imagine that a 20k mile oil change is not simply based on what is best for engine wear alone.
I suppose that 20k mile interval has been come to by a whole multitude of factors including environmental awareness/regulations, cost to owners/fleet managers, public/media relations etc. That's not to say that 20k intervals are in anyway harmful for an engine, but perhaps they are not 100% optimal for engine wear? That's not to say that people/American's that change their oil every 5k miles are doing it right because, again, from what I know, modern synthetic oils are capable of doing far more than this and still retain all of their properties. Short oil services are so unnecessarily wasteful both economically and environmentally. I figure 10-12k miles is somewhere in between a manufacturers 15-20k mile interval and the American's 5k oil services
I do understand that longlife lubricants have come a long way and I am completely ignorant to the finer details, so I could be way off the mark, but it's just my (un)educated look at it.
FWIW, on my M3, I do an engine oil change every 10-12k miles or so as that is my annual mileage.