Not being smart mate but no oil leaks would scare me a lot when talking 2 cycle Detroit engines; that is a good output from the Denning I would say.
I am pretty meticulous with my '92 Silver but it still has some "weeps" rather than leaks.
Have worked on many, many GM & Detroit 2 strokes, never leak oil if the job is done right the first time. Sounds like it could be the perimeter square section O ring, make sure everything is squeaky clean and don't use ANY sealer. You also have air box drain tubes in the base of the block on both sides which always have a bit of black oil coming out of them with a small amount of air.
Just had another thought, if it has done a lot of idling it could be unburnt fuel or maybe running too cool because the thermostat has been tossed away or isn't working. Put it to some hard yakka and don't leave it idle, no more than 5 minutes.
I have one of those in my Tourmaster, was oil all over it. Took it to Cameron at CPF and he steamed cleaned it, replaced a few bits and pieces and no leaks.
Just before last trip got them to do a service, about 9 K's on engine oil, used NO oil in that time, so NO just because it is a DD, it doesn't mean it has to leak !!
84 Austral Tourmaster with 6V92 and now 7 speed Eaton-Fuller, converted to motorhome "Vanishing Point" after a favourite American movie.
3 Kw solar 800 Ah Lithium house battery pack, all engine cooling done by the sun. Water injection for hot days and hill climbs.