Some smaller units may have different type screws and may be on either side,
all are usually marked.

The first screw you want to remove is the bottom drain.

The oil will not come rushing out because you haven't loosened the top vent screw yet; no big deal if you jumped the gun on that one.

We want to look for water in oil and "metal sawdust" in the oil.
Both are good early indicators of problems to come.

Oil is still barely exiting the drain, so what ya'll waiting on?
Remove the top vent screw and let it drain into a pan.

Both of the drain screws have a gasket or seal, but half the time it stays in the recessed areas in case.

Be sure it's there at refill time!!

If you observed over a tablespoon of water in the oil or a large amount of metal shaving, a couple more checks are in order:

If your lower drain plug doesn't have a magnet attached on the end, ("round as a tooth-pick and 'bout long as Abe's ear on a Five") then you could probe the lower hole with a small magnet and see if you retrieve any larger chips or fragments of metal.

Chunks and chips warrant a tear-down and look-see.

Some twinkling shavings in oil could be normal "wear in" of gears but nothing larger should be ignored!

If you had water in the oil, or a lot less oil than is required to refill case then a pressure and vacuum test is the ticket to less trouble.

I've got a gearcase vacuuum tester and a pressure tester right here at the shop that cost about 100 bucks a toss; you're welcome to stop by and use them anytime.
If you can't stop by, then you'll have to use your imagination on this one.

OMC units usually hold 11-16 psi for half an hour and a steady 20+ inches of vacuum.
Most Merc units generally start leaking at shift shaft around 5 or 6 psi but should hold steady for 30 minutes after that.
Rotate all shafts some during test.

Most auto mechanics (and auto parts stores) have a small hand vacuum pump that can be adapted easily to pressure and vacuum test a lower unit.
I think it's brand name is Mighty-Vac .

Are you wondering why a pressure test and vacuum test is necessary?

Each shaft-seal area has two seals, back-to-back, or a newer style double-seal:

The outside seal prevents water from entering case.
The inside seal prevents oil from leaving case.

Don't make one test and think all is okay!

To re-fill your gearcase squeeze tubes of reccomended lube into lower hole until it comes out of top hole.
Have your buddy hold his finger over top hole while you switch tubes and replace drain plug.
This trick kinda airlocks lube in the case.
Now you've got your friend a good excuse to hang around your boat and not mow grass for an afternoon too!

I'll try and get a page together soon with gearcase capacities for most models and sizes of inboards and outboards.

Hang in there or just check back later..