Passage Summary: Since a test showed that cheap oil protects engine parts just as well as expensive oil over 6,000 miles, there is no reason to pay more for the fancy brands.

Conclusion: Inexpensive motor oils represent the best value for consumers.

Reasoning: A 6,000-mile study demonstrated that all oil brands, regardless of price, were equally effective at preventing wear on the most critical engine components.

Analysis: To weaken this argument, we need to find a reason why the study's results might be misleading or incomplete. Perhaps 6,000 miles is too short a window to see the long-term degradation differences, or maybe the study ignored other vital factors like fuel efficiency or the buildup of engine sludge. If you're a car owner, you know that 'wear on pistons' isn't the only thing that keeps a car running. Look for an answer that introduces a relevant variable the study failed to consider.