A good point, I guess the air has to be quite badly polluted to explain the contamination..? And why isnt the dirt trapped in the air filter then?

According to a Volvo mechanics here, cleaning solves many of the ETM cases (but obviously not all) so the contamination seems to be the main issue.
As far as I understand, the crankcase ventilation passes through the throttle body. Could it be crankcase fumes that are causing the problem? If so I guess both oil quality and oil change interval could be involved. The amount of crankcase fumes would probably increase considerably as the engine gets worn?

Or is it simply combustion fumes flowing backwards when the engine is stopped? I have seen "gumming" in the carburettor on some cars I have had. The gumming was above the fuel nozzles so I find it hard to believe it was caused by the fuel. I have always thought it was caused by combustions fumes when turning off but maybe crankcase fumes is a better guess?

Any thoughts?
Rgds
Erik