In general, a lower viscosity oil will be better for fuel economy, and also in preventing wear when a cold engine is started.
So a 10W-30 will be better than a 15W-40 in that respect. However the thinner oil may offer less engine protection at really high temperatures.
Best bet is to stick to what is says in the handbook, paying particular regard to the ACEA rating (ie E9, or whatever) and the SAPS content…if the engine has a PM filter, then a High SAPS oil will permanently clog it.