[SolOmoN]

Up. The transom is designed for the weight, I would venture to guess the oil tank and cap is also designed not to let any product leak as long as the cap is secure. You could always put a oil absorbent diaper down if you are concerned with leakage. Trailering the boat with the motor up is more stress than sitting static in the water and that's no problem.

If its a local secure lake/pond.... and you are not worried about the prop getting pinched then there is no reason you could leave it up. If growth is your main concern and you run it every three four days through out the season then down would be no problem either.
Just my opinion and you know how that goes....
sol...