If you're not sure of the level, you might as well change it. The Mercury High performance gear oil is about the best you can use for the money ($10.00/qt.), and it's easy to change.
You will need to get the little pump that Mercury sells that fits on top of the quart bottle. It's also about $10.00.
Pull the bottom drain plug out, just below the propshaft and then pull the vent plug under the cavitation plate. Let it drain good. Once it's empty screw the end of the pump into the drain and pump it up until it runs out the vent hole. Put the vent plug back in and trim the motor up to about 30-40 degrees and unscrew the pump in the drain and re-insert the drain plug.
That's it. The longest part is waiting for the old lube to drain out. :wink: