I have always changed the oil when I winterize it.
As far as draining, I manually drain poke all the drain holes and then manually fill with either -100 (-45 freeze protection) Marine AF (if I'm too lazy to mix up my own) or, I mix up 4 gallons of Sierra PG antifreeze 50/50 with water, that gives a -26 freeze protection (plenty good for our winters). I do not use either the -50 or -60 since they get hard (but don't expand) at between +7 and zero. Even West Marine in their recommendations, advises that -50 and -60 are for winters where it doesn't get colder than 20 degrees. I don't want an antifreeze that will get hard in a cast iron block, even if it does not expand. This point is not well understood by do it yourselfers. I do think that adding antifreeze with corrosion inhibitors helps reduce corrosion, just like spraying trailer leaf springs with Corrosion X does, but you need to use the right stuff for your climate. Remember you DRAIN For freeze protection and add A/F for corrosion protection. The way I do it is:
Put the drive down, start engine on the muffs. Warm it up, change oil and filter. Then remove the plugs, disable ignition briefly crank over engine, check for water in cyls. If none put in old plugs, run engine again and fog through carb if a carbed engine. If not follow manufacturer's instructions for fogging mix in fuel filter or remote tank.
Once fogged, let it cool off. Then remove: 2 block plugs, poke holes, 2 manifold plugs, same, let it drain. After draining, replace plugs, put some Permatex Aviation sealer on the threads (prevents corrosion and leaks) Then disconnect: big hose from thermo housing at bottom, this lets water drain from front circ pump, and raw water intake hose at thermo housing (Alpha, Cobra) or impeller housing (Bravo, Volvo). Hold it down in the bilge, to drain, then hold it up and fill it with AF till it runs out the drive water intakes. Reconnect both hoses. Then disconnect big hose at thermo housing, fill it with 2 gallons and a bit more of AF that will fill engine, till you see some AF spurt out the housing neck. Then reconnect upper end of that hose. The next day will change the drive oil check for water in the oil. Pull drive for winter storage check bellows for water, grease gimble bearing and u joints.
Batteries, I usually use em till they won't take a full charge, I have a battery capacity tester now, and they do come back to their rated capacity, one batt is 5 years old the other is 7. In spring I'll test them again, if not good I will replace both. Also going to replace the old heavy direct drive starter with an ARCO PMGR starter, they draw less current and spin the engine faster.
In spring after the first start up, I take out the plugs used to fog it, check for water in the cyls and put in my newly cleaned plugs. Grease steering, drive shift linkage and re-install the drive. I usually get 10 years out of the Cobra style bellows. The I/O is a good bit of maintenance but it is simple for the most part, engines are simpler than outboards by far.