Motor oil serves to lubricate engines and thus retard engine wear  A study was conducted to assess the effectiveness of various brands of motor oil by using them in taxicabs over a 6, 000-mile test period  All the oils did equally well in retarding wear on pistons and cylinders, the relevant parts of the engine
Hence, cheaper brands of oil are the best buys
Ability to retard engine wear is not the only property of motor oil important to the running of an engine.