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. Which one of the following, if true, most weakens the argument? ['The lubricating properties of all motor oils deteriorate over time, and the rate of deterioration is accelerated by heat.', 'Cheaper brands of motor oil are often used by knowledgeable automobile mechanics for their own cars.', 'Ability to retard engine wear is not the only property of motor oil important to the running of an engine.', 'Tests other than of the ability to reduce engine wear also can reliably gauge the quality of motor oil.']