Let's examine the impact that answer choice B has on the argument. Imagine that there are 8 (random number) tests we can conduct to test the motor oils. Simply stating that another test exists does not weaken the argument. This answer choice would be more appealing if it stated that the test we conducted was subpar and that a better alternative existed.
As for answer choice E, if there are other properties of motor oil to consider other than the ability to retard engine wear, then it is not enough to only test this quality and then conclude that cheap brands of oil are best. This weakens the conclusion because now it is clear that the argument did not consider all of the relevant information.
