Not sure the negative impact of over filling however I would suggest they provided an upper limit for a reason.
Seeing as you refer to a dip stick I assume you are referring to a Volvo or Omc. On my Volvo drive, there is a vent screw behind the plastic shroud where the shift cable connects to he drive. This vent also acts as your max fill indicator. If you open this vent hole / remove the screw the drive will drain to the appropriate max fill level. Removing the dip stick will help the oil flow quicker. If you are having a hard time locating the vent screw, just remember that the bottom of the vent hole will be at the same elevation on the drive as the max fill line on the dip stick.
Happy boating!
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2004 - 288 Vista
Twin - 5.0 Volvo Penta GXi DP
