Not an oil expert here, but I doubt you'd hurt it using the synthetics.
Don't know how many hours are already on it [real low you may have benefits; real high, it may be around the corner from a rebuild either way]. Assuming "average" and you're not planning on really putting on hours or abuse (and don't mind having different oils on the shelf), I'd stick with whatever you think it was running.
Now if you were talking say 50 years old, there's a slim chance you might regret using anything other than non-detergent....but that's another topic.