Good evening!
I understand your problem Kiwimodeller!

I had the same problem ,... before!
But , and it will be discussed heavily for sùre, I personnally do not put too much oil in my motors. Virtually nothing.
Firts: all my boat are set with an oil separator and i can by pass its outlet to the cheminey or on the side as I wish.
When I warm up my motor, this is where the most of the oil is coming thru (the top of the oiler). I am connected on the side at this time. When steam comes out clean with virtually no oil, I can leave it this way or connect onto the cheminey.
Normally, when it is hot and if you are not too generous with lube, your boat should stay clean. At least as much as a steam boat could be.

I hope that I have been of some help!
Regards!