These recommendations generaly apply to run of the mill "petroleum" oils. They are not considering synthetics. The 5W-30 is preferred for it's cold cranking and cold flow abilities, because it starts with a somewhat thinner base oil and may have more pour point depressant additives. Since the 5W-30 uses more viscosity index improvers to "prop it up", it is less shear stable. The 10W-30 should be more shear stable and hold it's viscosity better under high temperatures and load. The best of all world's would be to have a good 0W-30 synthetic. Stressing the oil pump should not be an issue going to a 40wt. Many here like the 0W-30 German Castrol, where 30wt. is recommended. It's a thicker 30wt.