Edit added comment in response to
_Whether we agree on extended service intervals, or not, the mechanic if they were asked to indicate other than the manufacturers recommendation is unlikely to do so. It is a potential liability that would be expensive and complex to defend if a customers vehicle subsequently failed outside the OEM recommendation. _
We are all free to make alternate decisions and only book the next service to suit. In doing so we take on the risk. And unfortunately general advice that may come from a good source needs to be qualified to ensure it is appropriate for your individual needs. Similar to the qualifier on financial services information.
End of edit
A high risk business strategy. Charging for oil that is not replaced. Potentially a criminal offence?
Even the highest quality synthetic engine oils after 10-15,000km will appear, smell and possibly even feel very different on the dipstick when compared to a fresh new oil sample.
Having a reliable honest mechanical service provider may be the best solution for the average consumer. Knowing you have one? The state based motoring organisations do list approved mechanical service providers.
P.S.
On not changing oil over a long period of time. Changing the oil filter is not a solution as the filters only remove coarser contamination. Providing the oil level is regularly checked and topped up, anecdotally it can be many tens of thousands of kms before there is any other evidence. Often gummed up oil control rings, increased oil consumption and a smoky exhaust. Increased and obvious rattling noise from the top end (cams and valve gear, excuse simplification) is also a common symptom of lubrication neglect. For turbo charged engines the symptoms may be more severe and permanent. And who knows what else?
