All engines: Prolonged no-load operation of the engine may cause it to blue/gray smoke at low rpm with an associated increase in oil consumption. For every 3-5 hours low load operation (trolling) or no load operation (running the fridge or charging batteries) should be countered by 10-20 minutes near maximum continuous rated rpm.
What is the 1GM10 really rated at, seems we call it an 8hp or a 9hp?
The owners manual says this:
Continuous rating at 3400 rpm = 8hp
One hour rating at 3600 rpm = 9hp
So now whe know.
Note, the open vent hole must be above the level of the outside water, the instructions that come with the Vetus siphon breaker are very clear to read.
Pictures of the Yanmar GM10 and Vetus siphon break are by R McBride, the camera was a Canon G11 on a standard setting.