Mine is a 1996 Mariner 75 with the rectangular type oil tank filler , only leaks when the boat is being towed and the outboard is in the fully up position , I presume it is the seal inside the oil tank lid at fault or the lid has gone out of shape ,I might get a new seal as I tend to loose 1/2 litre of oil on the last 300 mile road to scotland.
it dumped this into the bilge
Alan.