I'm not an oil expert but... even though synthetics are proved superior in every way, I have always been told to change the oil before layups so the engine isn't sitting bathed in oil that has accumulated corrosives and acids that are by-products of combustion. I motor less than 100 hours per year and change oil just prior to annual winter layup, so the synthetics will provide better lubrication, but is it worth the extra money when the change interval isn't extended?

You decide. The engine makers and mechanics don't seem to push synthetics, so I use Delo or Rotella. (Except I found out that recently Audi recommends synthetic for their cars).

Also, I never mix oil types if I need to add oil between changes because the additives in each may be different, and additive effectiveness may be altered by dilution.