My OMC manual said the same and I felt the same as you. What I do is disconnect the raw water hose at the 'stat housing and fill that hose with -100 (-50 freeze point) marine antifreeze till it runs out the water intakes on the outdrive. Doing this 20+ years and never had a problem.
Yours is different due to the impeller being on the engine but you can do th same thing. First mark the hoses so you don't mix em up. Then disconnect the hose coming from the transom to the impeller housing. Hold it down to drain it and then hold it up and fill it with marine AF, till it runs out the water intakes. This also flushes water out of the P/S cooler on the back of the engine. Next I would disconnect the hose going from the impeller housing to the 'stat housing at the stat housing. Then fill it with marine AF till it runs out of the impeller housing. Reconnect the hose from the transom to the impeller housing, and then keep filling the hose from the impeller housing to the 'stat housing till its full then re-connect.
If you get close to zero *F, do not use -50 or -60, I would use only -100. Those will get hard at about zero, which you do not want in a cast iron engine!
Hey, so earlier today I had followed your instructions from last year combined with a few I added (I added 4 & 6)
1) Lowered outdrive
2) Removed 5 drain plugs and used paperclip & pick tool to ensure unblocked
3) Disconnected bottom end of big hose from tstat to circulating pump and drained
4) Disconnected bottom end of hose from tstat to impeller and drained
5) Disconnected hose from transom to impeller and drained, then lifted this up
6) Opened Impeller housing and drained water, inspected impeller to check condition , put impeller housing back together
7) Lifted hose from transom to impeller and filled with AF until it came out the water intake on the outdrive
8) Put drain plugs back in
9) Reconnected both hoses to water pump
10) Reconnected bottom of big hose from Tstat to circulating pump
11) Disconnected top of big hose from Tstat to circulating pump and filled with AF from the top until it came out the tstat housing and then re-connected the hose
12) Disconnected each of the feed hoses to the manifolds and filled with AF until it came out the back
In your response earlier you mention "Next I would disconnect the hose going from the impeller housing to the 'stat housing at the stat housing. Then fill it with marine AF till it runs out of the impeller housing".
Is that something I should / could still do at this point with all the AF I already have in there? I'm not sure if I disconnect that hose now if all the AF I have already put in will come flowing out. Probably overthinking it considering it is just for anti-corrosion and I think what I did earlier would have gotten all the water out.
