I thought THIS site was for heating, too?
I'm a steam expert, so they tell me, and I stopped posting over there when Constantin and the Viessman fans began tromping on every post from my customers.
I won't bore you with the details of every incident, but that site is turning over to mostly homeowners advising homeowners.
Back to HVAC on THIS site....
That boiler could use a surface skimming to remove oils from the top of the water. Draining the water out only coats the walls of the boiler with the oils.
Draining out the dirty water is a good idea, it just won't remove the oils.
Make SURE the piping diagram in the manual illustrates YOUR boiler piping, or the water level will be all over the place. If your equalizer is undersized (or ANY pipe), the water will head out.
Make sure the boiler isn't TOO BIG, or it will make steam faster than the steam piping can handle and the water will leave the boiler out the return piping.
Make sure the boiler itself isn't overfired (Fuel pressure and nozzle or orifice size).
If you have wet returns, be sure that they are open, and not partly plugged.
It sounds like oily water, to me.