Hi, I'm also interested in the discussion, having also found small traces of oil before the throttle body.
I had this problem before with a turbo that leaked oil externally, but thinking that it also lost inside.
After replacing the turbo and cleaning the throttle valve and the duct, after about a year I found traces of oil in the duct and in the throttle body.
I think it's a problem that comes from the oil vapor storage container.
I would be interested in understanding how it works and whether it is possible to do maintenance or examine this non-return valve.
The problem is that I don't know where to start and I realized that the container is in a somewhat difficult place.