Your best bet to keep corrosion down is to change the oil prior to storage. As others said, it will get rid of the contaminants that have built up through use. That said, oils have additives that neutralize the acids and contaminants in the oil so even if you change it earlier, there shouldn't be any harm. Many people change the oil as part of 'spring maintenance' and don't touch it again until the following spring (and some will wait even longer). Modern engines are better built than the ones 30+ years ago and have much tighter tolerances. This means less break-in time and less blow by with the accompanied oil contamination.
Synthetic vs. dino shouldn't matter unless you are pushing your engine hard. Synthetic can handle higher temps without breakdown but your engine shouldn't be getting anywhere near hot enough to worry about this.
Tim, Tara and Canyon - 'TnT' (& C now...)
2015 Thor Outlaw 37LS w/FJ Cruiser Toad
Columbia 400, 1967 Corvette, KTM 350 EXC-f, TTR 125 LE, XR50 for toys