Well, I removed "setChannelId" for API26+ (createNotificationChannel remains). Works in Oreo, if app is in foreground.
In background ...
Service.StartForeground in Oreo is limited by 5 seconds afrer Service_Create, To bypass this limitation I moved Service.StartForeground (...) to Service_Create subroutine.
In firebasemessagingInstance_MessageArrived event the program saves notification and restarts the service (StopService (Me) .. StartService (Me)).
Remains background problem in Android 4.