The question directly asks for something that weakens the argument, making this a Weaken question.

The author concludes that motor oil should be changed every 3,000 miles, because after 3,000 miles it is only 50% effective.

The author is assuming that, once oil becomes only 50% effective, it needs to be changed. The correct choice will counter that assumption, showing that there it may not be necessary to replace motor oil when it becomes only 50% effective.

(C) matches the prediction and is correct. If 50% effectiveness is enough to match the performance of fresh oil in preventing engine wear, this makes it less likely oil would have to be replaced at 3,000 miles.

(A) is irrelevant. The author is not concerned with why the oil loses its effectiveness but whether that loss warrants changing the oil.

(B) is irrelevant. It does not matter how long it takes to reach the 3,000-mile threshold. The only question is whether or not the oil needs to be changed at that point.

(D) may be tempting, but it is irrelevant. A choice like (D) that questions the motives for the research may seem to weaken the argument. However, the evidence in a Weaken question can never be challenged - only the central assumption. Who supervised the research is not an issue. The argument is only concerned with the reduction in effectiveness and the recommendation to replace motor oil after 3,000 miles.

(E) is an irrelevant comparison between two types of oil and has no effect on the argument.