I've heard it's good to run the engine out of fuel completely and store for winter.
Also, I may do an oil change to avoid dirty oil staying in over winter.
Is it worth adding any supplements?
Doesn't the engine need a bit of fuel or something left in the chambers to avoid rust?

 

I run my snow blower and lawn mower dry before off-season storage but not all do.

 

I've always run mine dry also although some claim it's better to keep a full tank with stabil added to the fuel.

 

Can I use seafoam for storage instead of Stabil?
Also, the tank has a max fill lne, which would leave a bit of room in the top for condensation. So, should I overfill it and then just siphona bit off in springtime?

 

Since burning only premium fuel in my small four stroke engines I have done nothing except turn off the fuel shut off if there is one and cover it with a tarp.
Two stroke I will use premium fuel and put stabilizer in the last fuel tank then dump the tank into the next season's engine.

The difference is that when regular fuel sits for one seaon the tank smells like c*ap and with premium smells like new.

 

Since you list as non U.S., you probably do not have to deal with ethanol gas. That is a plus for you. Warm the engine and change the oil before storage. Add Seafoam to the gas. Run it long enough for the Seafoam to get into the carb. A full tank of fuel reduces condensation in the tank over the winter months. Finally, use a battery maintainer to keep your battery charged and eliminate the chance of it freezing.

 

Regular gas in Canada contains 10% ethanol.