how long had the motor sat, before you fired it up??
was the fuel fresh??
i had a similar scenario with a couple of motors i have here, including the one on my tinny.
if the motor hasn’t been started for a long time, the petrol evaporates from the oil and the excess oil will exit out exhaust.
i’m not a fan of the flush muffs, especially when a cheap plastic drum/bin can get full immersion, and don’t require running water. using a drum or bin, won’t annoy the neighbours as much, as the exhaust is underwater.