Dave,
I suspect that the additive levels in RL 5W-40 and 10W-40 are the same. The only difference is that 5W-40 uses some amount of viscosity index improvers and 10W-40 most likely uses none (or much less than 5W-40). As you might know some VII will shear down, leading to loss of viscosity and other basic properties of oil. Probably RL uses very stable VII, but still... Personally I would pick RL 10W-40 over 5W-40 for sure. If it gets rather cold where you live (below 0F or -20C) I would consider using 5W-40. You might want to try Delvac 1 5W-40. Although I'm not a fan of Castrol, their 5W-40 seems to work fine in Euro engines. Castrol's 10W-40 would be OK as well. If you have a choice Redline, Mobil 1 Delvac 1, and Amsoil are very good candidates.
Hope this helps.
Regards.