http://www.bobistheoilguy.com/ has all sorts of good information about lubrication.
If you need to use that wide of a viscosity spread, I suggest synthetic. 10W40 standard oil has at least 17% viscosity improvers (VI) and 10W50 has even more. 10W30 uses only 11% VI. Those VIs are taking the place of oil. Synthetic oil needs a lot less VI. Many years ago, General Motors would void their warranty if you use 10W40.
I base this information on what I gleaned from a petroleum engineer that I had the privilege of working with.
Richard
