First a simple possibility: was the engine carried/leaned/stored on its side for any period of time? 4-strokes are sensitive to which side is "down". If so, find the manual for that year for instructions on laying the motor down for future reference, but there is no real problem - just clean up the mess, check the level, and carry on.
Could it possibly be coming from the lower unit? Doesn't sound like it is, but if it is, replace the seals/gaskets around the fill & vent screws.
Does the engine run? Any other symptoms?
A problem with the piston rings could lead to that problem, but you would have other issues (does your engine oil in the oil tank smell like gasoline?)