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Did the annual winterization on the 99 FW Sundowner with 5.7 Duo Prop outdrive today. We used the Camco flushing kit with the raw water pump adaptor to run through about 5 gallons of -100 non-toxic antifreeze once the thermostat fully opened. Instead of leaving the system full of antifreeze over the winter, we elected to completely drain the system once we ran the antifreeze through and to leave all the drain plugs open. The flushing kit worked really well, you just need to ensure the adaptor is connected to the suction side of the raw water pump on the front of the engine and not the output side. After dumping some fuel stabilizer into the gas tank, we fogged the engine while it was running through the antifreeze and then changed out the oil filter and oil on the engine and outdrive. As others have found out, the small oring on the oil drain plug for the outdrive was pretty chewed up and had to be replaced. Will do the new spark plugs and plug wiring before putting back into service next season. Will get the boat shrink wrapped in the next week or so and call it a season!

I've said this before, but at the risk of beating a dead horse...
the value of using antifreeze, is purely in reducing corrosion. If you drain properly, there is no need to use antifreeze to prevent freezing. In that use its more like something superstitious. So if you want to get your money's worth out of using antifreeze you should follow Merc's and OMC's instructions and:

Drain everything including hoses, properly.
Then back fill the engine, manifolds and raw water intake lines with the best -100 marine antifreeze with corrosion inhibitors and leave it in. It reduces corrosion by keeping air out and by the use of corrosion inhibitors. Remember raw cast iron rusts like crazy when exposed to moist air. When I start up the engine in the spring I do not get rusty water coming out of the exhaust, because I have -100 marine antifreeze in there all winter. That tells me that it works.

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88 Four Winns 200 Horizon
4.3 OMC Cobra-4bbl
2002 Walker Bay 10/2012 Suzuki 2.5
2008 Walker Bay 8

1998 Jeep Grand Cherokee 4.0/Selectrac
2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II

I'm with Lou on this one.
I live in Australia and I too leave antifreeze in my block - solely for reduced internal corrosion over winter.

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2011 Four Winns H180 SS.
Aussie built Mustang 2800.
SOLD - 1997 Four Winns Horizon RS

Ya, understand the concept of leaving it in to prevent corrosion. Have heard several others say running the antifreeze through in addition to thorough draining could provide an added layer of protection should any water--however slight-- be left in the block exhaust system or oil coolers after draining. Probably overkill as far as freeze protection goes but with temps here in winter consistently down in the low to mid teens I don't want to take any chances.