Why not all use longlife oil and the ones that do the oil quality is managed by the cars electronics and will inform when it does need changing.
Mine is serviced yearly at 10k miles or 12 months which ever comes sooner, it is well looked after by me with spotless paintwork and low mileage, whoever buys it after I return it will get a good buy.
Like everything you do your research ask the right question etc motability car buying is no different from any other in that respect and yes there are some bad ones out there but no different to the car market in general that too has bad apples.
But as I say if you do the research good buys can be had as they are generally well maintained and as the managing dealer is being paid to maintain it there is no reason why they would skip on service schedule’s or repairs and indeed by viewing the V5 you have a good idea wo maintained it and again do your research on that dealer.