I have got a used boat with a 1995 Mercury 115HP 2-stroke motor on it.
The motor has an oil-injection-system. This means in theory I don't
need to mix oil with fuel. But when I examine the fuel in the boat, I
find that the ex-owner has already mixed oil into the fuel. I am
wondering if the oil-injection-system really works or not. I would
like to find a way to check if the oil-injection-system works or not.

I supposed I can run the boat and keep an eye at the oil level in the
oil reservoir (knowing that the fuel already has oil and I don't need
to worry about running the motor without oil). If the oil level
gradually drop, this means the oil-injection-system seems to be
working, and I can feel free to add fuel without pre-mixing with oil.
If the oil level stands still, this means the oil-injection-system
fails, and I should pre-mix with oil and have the motor serviced. Does
this sound right?

And yes I need to carry more spare spark plus because the motor will be
running very rich with oil.

Thanks.

Jay Chan