A conflict of interest is a confluence of circumstances, not a particular behavior. The most widely-used definition is:
A conflict of interest is a set of circumstances that creates a risk that professional judgement or actions regarding a primary interest will be unduly influenced by a secondary interest.*
*Lo and Field (2009). The definition originally appeared in Thompson (1993).
For example, if a doctor prescribes a particular medication, and he happens to receive favors or money from that particular pharmaceutical company, he has a conflict of interest, even if he is prescribing the medication for the right reason.
Likewise, if I get a discount for personal use form a vendor, and then I recommend that same vendor for professional work at my job, I have an inherent conflict of interest, even if this is the best vendor for that particular service. I might be doing the right thing, and I might even be doing it for the right reason, but there is a conflict of interest present that I should avoid by using an impartial bidding process or by letting someone else choose the vendor who doesn’t have a conflict of interest.
So I’m not saying it’s necessarily wrong to recommend 5,000 mile oil changes for reasons other than the oil change being due, I’m just saying that, because it serves the mechanic’s or shop’s financial interest to shorten maintenance intervals, people should be honest about their reasons for doing it.