[pentapenta]

Hi everyone, it's sea water cooled.

I've found the way to prepare it for winter, but the guide assumes that the engine is running.

So my question is, do we replace the oil and oil filter and try to start up the engine, or do we just see if we can do a compression test while powering the starter with a battery and plugs?

[Gelis]

Your problem can have different couses. Due to the fact that this is an old engine which normal have seawatercooling.
The watercooled exthousemanifold can have been damaged due to rust, this will give water in the cylinders and further to the oil.
The head can be broken due to rust.
The headgasket can be brooken due to overheating, common problem due to rust in the coolingchannels.
Anyway don't try to start the engine if you have water in the cylinders, water can't compressed.
This engine have also a watercooled oilcooler, this one can also have a problem
The the sealing for the impellerpump (seawaterpump) can give water in oil if both of them is worn out.