Ok, here's the straight scoop with oil...
The weight ratings 10-40, 20-50 etc were created to specify how an oil reacted to temperatures with viscosity modifiers added. In cold temps, oil would get too thick, then in hot temps the oil would be too thin, thus the multi-weight rated oil.
However, synthetics are TOTALLY different and really don't follow the same rules as dino juice. For instance, 20-50 Amsoil synthetic has a pour point below -40 f! That's much lower than any 10-30 dyno juice on the market! In addition, since synthetic has MUCH higher resistance to thermal breakdown 20 weight synthetic "acts" thicker than any 50 weight dyno juice at 500 f. So why does synthetic even have multi-ratings?
Well, one reason is for the size of the oil passages, tolerance of bearings, etc. Where the oil rating gives the user/manufacturer an idea of how fast the oil will flow and how resistant it is to being "squeezed" out of place under pressure. Thus, 10-40 will flow faster at a given temp than 20-50, but 20-50 will resist being squeezed out due to pressure. Therefore, when running full synthetic oil you don't have to worry about the whole summer/winter debate. It's best to use the grade recommended by the manufacturor unless you've made modifications.
I'm installing an oil cooler on my bike, which suzuki didn't take into account when they spec'ed 10-40, so I'll likely run 30 wt. full synthetic year round since my oil will be cooler than the design spec under all operating conditions. I could go on here, but I'm afraid I'm explaining how to build a watch at this point when somebody only mentioned what time it was...
Bottom line:10-40 full synthetic is MORE than sufficient for the hottest temps your Thanksgiving turkey will encounter.