Well there always looks to be gallons of oil in my bilge, dont matter how often I clean it out. Then again I nearly never put oil in the engines. If you look closely the oil is only 1-2mm thick and the rest is water. On the other hand breathers for some reason soon get clogged so the man might be right.
Tell him to change the filters and clean the bilge before you/ others go to look at it again. But like I said oily looking bilges are quite normal, well with shaft drives. Out drive boats is another story but still might get rain water down there and with a bit of spilt oil it looks allot worse than it is. More investigation needed. But dont knock it out of hand.
If they are Volvo's then VP say change the filters when oil emits from the filter they are then blocked, fit new filters, if it still spews oil then chances are that there is an engine compression problem 41 series generally ok but have recently seen one that had to be lifted out, new liner and piston.
On any engine that gets a build up of sludge in the breather I would at first think about back pressure .If your very interested in the boat get the engine compression checked .
I dought that the engine is being over cooled ,that could cause it as well.
Head gasket, if the gasket is blowing that could pressurise the sump add water to the oil and blow it out at any point possible, a compresion check should show if there's a problem .
Did you have a look at the exhaust to see if there's to much moisture present , and is the engine running on idle with an even beat .
Even after cleaning the oil filler cap you will soon see the sludge back again if there's a problem thats not been fixed.
Another point some engines will if left for long periods drip oil through the crankshaft seals ,think about it ,the oil thrower is not working ie not turning so the oil eventualy seeps out.
Cheap oil could do it as well or the wrong viscosity of oil.
I agree with whats aready been posted as well , it could be as the bloke says but !!!