I have checked oil does not come cause of blow by, as it does not exist as Klaus at wahl-spezialkolben.de made a superb job with pistons.
After measuring valve movement i noticed it is same as actual lift. Seals are not broken, so my hypothesis is that retainers do come too close and create pressure which push oil between sealing and valve stem and to intake channel.
Cause of this two mm:s must be found. One comes from valve guide seals which are grinder one mm off and one from grinded down retainer.
Hope this simple fix do solve unpleasant contamination problem.
********
2.2.2020 Edit: ...Actually it does not, as i noticed new bronze valve stems are already worn out, just after 10000km. Valve head lifted off today. Research will continue.
********
![Bild](http://ital-web.de/forum/userpix/1733_Screen_Shot_20191216_at_221038_1.jpg)
![Bild](http://ital-web.de/forum/userpix/1733_Screen_Shot_20200126_at_151740_1.jpg)