Pulling the plug on your lower unit will extend its life. So routinely check gear oil for the water. You'll need a long handled slot edge screwdriver, the jumbo shank provides the extra leverage you'll need to get the drain plug  loose without buggering up its edges.

For best results check gear oil a day or more after the engine has been run, this to allow water to separate from the oil. Oil floats on water, even when there's only a drop or two. Barely crack the lower drain plug screw and see what issues forth.

A few drops of water means you ought to check more often in case the problem gets worse. However, an ounce or more means a seal has failed catastrophically, or is about to.

Undetected in its early stages, the problems brought on by a leaking seal cost hundreds of dollars to repair. In a worst case scenario the repair bill can run nearly two-thousand dollars in parts and labor.

 When changing lower-unit oil, examine the spent oil for signs of overheating. Dip you finger in the oil, smear it across a finger. Sniff it.

The telltale clue of burned oil is an unmistakable, burned smell and very black oil. Normally its yellow brown.  Milky brown means water mixed in with the oil by the whirling gears.

Obviously broken chunks of gear teeth or bronze bits also mean serious mechanical damage. Dunk a magnet in the oil, it will pick up metal particulate you can't see with the naked eye.

When buttoning up, make sure each gear case drain plug wears one and only one gasket. Old gaskets are infamous for hiding inside the housing, and the do-it-yourself mistakenly believes the gaskets is lost. Two gaskets don't seal. Neither will none.

Refilling can be a problem if the boat is in the water. Spilling even a drop of gear oil can cause the wrath of the USCG to descend upon you. The remedy: Use a gear case pump and quart bottle of lubricant instead of oil in a tube. Also, keep an oil absorbent rag handy to dab up errant droplets.

Fill from the bottom plug until oil just begins to trickle out the top plug. Replace the top drain plug. Its vacuum seal will hold the oil in place long enough to insert the bottom drain plug.