An engineer friend of mine asked, "Why does Nikon use grease to lubricate?"

I think (to me) what is most important is where the oil is coming from. Is it coming as a residual condition of assembly, or is it coming from what should be a sealed system that is leaking. The former can be tolerated, the latter cannot.

And I agree that it does make a difference regarding what field the engineer is in. An IT engineer, or even a chemical engineer might not be as qualified as, say, a mechanical engineer ot an industrial engineer. But let's not dwell on the specificity of the engineering field, but more the appropriate question [by the engineer], IMO.