Oil in boat transom

I am getting oil in the transom of my boat. I have a 21ft Ranger with 250hp Yamaha. On the lake and when boat is pulled out of the water it is as clean as you would expect. However when I arrive home there is fresh oil in the transom of my boat. I cannot find any leaks anywhere. Someone suggested the oil tank on the engine was overflowing when the engine is tilted up. Is this possible? What would cause it to overflow? Thanks for any help.
Ralph

[boats.net]

Check that vent hose from engine oil tank is connected to air silencer in front of carbs, hose may be loose.

I found my leak today. Why it would only leak on the pull home surley escapes me. I had a small split were the oil line from the engine and remote oil tank were joined. I repaired it myself Thanks
Ralph