Actually the 10W -40 is fine in the summer. In the winter 5W-20 is a real good choice.
Most 15W50, 5W50 and 20W50 synthetic oils have pour points of -40f to -50f. Most conventional 5W20, 5W30 and 10W30s have pour points of around -25f. So if you opt to synthetic, you can choose any weight oil you want and still have a bike that will crank easier than a 5W30 conventional.
Now if you go with a synthetic 5W30 you can get down to -70f pour points.
My Guzzis will crank almost normal speed and fire right up even at -20f with a synthetic 15W50. JB