I have the same engine ('04 150 2-stroke), and oil used to siphon backwards from the main tank on the engine back into the resevoir in the bilge when tilted up. Fixed that with a check valve, but no leaks from anywhere.
I'm guessing the motor cover you're referring to is the black cover over the carb air intake.
How full does your main tank (engine) get and does it leave an air gap between the oil top level and the vent tube? With the engine up, the vent tube going to the cover where it connects to the tank should be high enough that an air space should seperate the vent tube from the oil, so it shouldn't be able to leak down to the cover. At least, that's how mine is configured.
The tank should have an oil full mark. Does the oil pump stop filling above this line?
A quick test is to remove the cowl and visually check the oil level after you've started the engine. The program in the computer runs the oil pump for 180 seconds after startup, or until the sensor detects the oil tank is full. Which ever occurs first. If the level goes above the full mark on the tank, then you may have a bad sensor or the sensor may not be fully inserted into the tank.
But like with markman, if you've got a leak from the cap (which would be under oil when tilted up), then that can be corrected with better fitting parts.
But just curious reading your post, it reads to me like you're adding Yamalube to the main tank on the engine? Not sure why you'd need to do that? And not sure if you're adding it to the sub oil tank (mine is in the bilge on my 2120), the connection with oil at the engine. So I'm just a bit confused. But that's OK. I get that way a lot.
I just looked at the diagram to be certain. Oil injects directly into the carbs on this model, so it shouldn't leak into the combustion chamber to foul a plug directly. I replace my plugs every season, sometimes even twice, and they always have a coating of oil so that may not be that unusual.
