The 4-108's 'famous' leak is at the rear seals
. It seems they all leak there to some extent or other. The mechanic who worked on the warped head
on mine said it's common and, except in the worst of examples, simply lived with (a lot of the fishers and shrimpers here in the Gulf have 4-107s or 4-108s in their boats so he saw a lot of them). It's generally controlled by an oil absorbant pad under the engine.
I'm not sure what you are describing by "Max to Min on the oil gauge" though; I assume oil pressure? That may tell you oil levels are dropping but it doesn't tell you how much oil you actually in the engine or have lost
. I chiefly rely on the dipstick as the most reliable indicator of the amount of oil in the engine (and oil loss over time). Are you losing a considerable amount of oil per the dipstick in the 20 hour runtime you describe?
On my 4-108 the rear seals seep. But it's surprising how just little oil seepage has the appearance of a LOT of oil in the oil absorbant pad (on my engine, in a pan beneath the engine), flung around the engine compartment and occasionally ultimately draining down into the bilge (where I have a bilge sock as a backup). Even just a few drops gives a considerable amount of oil film on the pad and the engine itself; and it doesn't take much to give a sheen on bilge water
That said, I've seen a bit of an increase in oil in the bilge of late - more than I think is accounted for by the rear seals. But there's no appreciable loss in the engine oil level per the dipstick. As already suggested here, I've already bought a can of degreaser. My plan is to first check around the engine where I can see some oil runs at fittings, and check for tightness (my engine mounts have a tendency to work loose over time, leading to vibration, and I'm thinking leading to things loosening up); tighten where necessary; degrease, and monitor