Secondly, there is the viscosity index (V.I) which is not related to viscosity. All oils become thinner when they become hot and oils with high V.I numbers remain more stable to give better protection. (Refer Chart in Viscosity Index)
"The wider the spread, the greater the amount of viscosity index(V.I) improvers in the oil. Although these additives serve a useful function, they can shorten engine life. When some V.I. improvers get very hot(as they do around the pistons), they decompose into a carbon-like substance that can make the piston rings stick or even break."