If anything, this might slightly strengthen the argument. If knowledgeable mechanics are happy to use cheap motor oil, this seems to support the author’s argument that cheaper oils are better buys than more expensive ones.

Without knowing how those other tests judge cheap oils, we can’t draw any conclusions. We would need something to differentiate cheap oils from other oils.

While it would be nice to know if cheaper oils deteriorate faster than more expensive ones, this choice just tells us that all oils deteriorate over time. On its own, this doesn't weaken the argument.

We don’t know which brand of oil was used, and whether it was expensive or cheap. Even if we knew, this case of a few individual cars and one brand of oil is too specific to have much of an effect on the argument.

This tells us that there are other important factors in motor oil performance, which the author hasn't told us about. We therefore can’t say cheaper oils are “better buys” based only on their performance in slowing engine wear, without knowing how they perform other important functions.