Most owner's manual recommendations assume non-synthetic oil, which will break down over time and "thin." If you're going to use full synthetic oil you can ALWAYS move to the next "thinner" oil. For example, if the OM says use 10-40, you can use 10-30 or 5-30 full synthetic. After about 50 hours, most petroleum 10-40s are no thicker than 10-30, and some 20-50's break down to something between 30 and 40...
As a general rule, thinner oil will give you easier starts, better mpg, and cause the engine to run cooler. Thicker oils generally offer more film strength, shear force & shock load resistance (for gears), less oil leakage, and in petrol only oils, longer life before the oil gets "too thin."
I'm currently using Amsoil 10-40, but am switching to Amsoil 10-30 on my next oil change. Both are full synthetic & wet clutch compatible. Just my two cents...but like Jay said, and others have said, there's an oil war here every so often and I guess we were due.