A number of members have explained that manufacturers' service intervals are intended to appeal to fleet buyers, who usually dispose of cars at 3 years or even at 11 months.

These fleet sales are a major influence, as far as private owners are concerned, most cars will survive to the end of the 3 year warranty period before engine wear/issues become a problem.

Oil is gradually contaminated with carbon and some of its anti-wear additives, viscosity improvers etc will gardually degrade too.

So for long life, annual/10,000 mile oil and filter changes are better for the owner. Worse for sellers of replacement engines/manufacturers' repair shops profits.

But do feel free to believe the manufacturers just have your best interests at heart.

I am interested in the reason people seem to think they know better than the manufacturers how often cars need servicing.

Car manufacturers specify long oil change intervals to make their cars more attractive when new to fleet buyers who want the minimum possible amount of servicing in the first 3 years of a car's life.

Car manufacturers do NOT specify oil change intervals to ensure longest engine life - as long as it's out of warranty, they have a vested interest in making engines last as short a time as possible!

Unsupported conspiracy theory

Not really, forget engines for the moment and look at 'filled for life' gearboxes. Those who don't change the oil usually suffer failures many thousands of miles before those who change the gear oil against the manufacturers advice.

Unsupported conspiracy theory

Since you know it all, just do your own thing and allow others to do the same.

You asked for opinions, you got them and now can do what you want.

Unsupported conspiracy theory

Really ?

First off, describing anything as a 'conspiracy theory' basically says to the world 'I've lost the argument, and since I've not got a valid point to make against it, and since everyone is against me, I'm going to put all those people who are against me into the same 'nutjob' pile as the JFK/Roswell lot.'

Real good way to go.

Got some news for you. Einstein's theory of relativity. Ever heard of it ? It's still a theory. As in, it has (so far) been impossibly to prove - though it's also been impossible to disprove too. All we have is a theory. And yes, we can't PROVE it as yet.

BUT ... what we do have is a large amount of observational evidence.

For starters, HJ himself. 40+ years in the motor trade. His THEORY is that these extended oil changes are terrible for engine longevity.

Then, dozens of independent garages - notably including ones who specialise in the marques that do a lot of longlife oil changes - BMW, Audi, Merc. Go on, pick up the phone. Call one of them, and ask what oil change interval they recommend. Ask why. Again, what they tell you is only a THEORY.

Then, the experience of people who've worked in oil formulation (me, for example). I'll tell you now, clear as day. Engine oil (especially so for diesels, due to the acidic combustion products) that has done 15k or more is basically worn out. That's a THEORY.

But, in spite of the fact that we can't prove our theory, it's backed up by observational evidence.

Want a conspiracy theory ? I'll give you one. You work for BMW/Audi/Merc/an engine rebuilders, in their new engines dept. If people start changing oil more frequently, you'll be out of a job.

That, or you're in fleet marketing, and are scared of prices going through the floor on cars that have done 60-80k on 2 or 3 oil changes.

No - OP had " I get the old "Oil OK" display light back on". I have had a 307 for 12 years and have never seen an OIL OK light. I have seen a level indicator but this isn't what he isdescribing, to me.

Most cars have no issues with the long service intervals and my conclusion is that the vast majority of those that do have simply been serviced using cheaper non manufacturer specified products.

It's certainly a possibility that cannot easily be ruled out. When the PD diesel engines were introduced across the VW / Audi / Skoda /.Seat ranges in the late 90s and early 2000s, there were a number of incidences reported of dealers (usually the retail type, not main dealers) not heeding the stipulation to use the required special PD oil, leading to some engines taking an early bath.

The term 'full dealer service history' is no guarantee that servicing has been done correctly.

But the important thing here is all these oils were rubbish compared to the fully synthetic long life oils that are used today.

There is no reason why 20,000 miles on these high spec oils would be an issue, my last BMW has an oil change every 2 years and never missed a beat.

It's not that simple - as a lubricant oil is actually much worse than it was in the 1970s - today's oil has far less zinc (damages cats) – the best last defence against metal on metal contact - and more of the oil's volume is made up of detergents, viscosity enhancers, emulsifiers, anti-frothing agents, alkalines, particle suspension agents and other chemical potions that do nothing for lubricity. Modern oil lasts longer chemically - as a lubricant it's not that great - modern engines are built so well, to such fine tolerances they can tolerate it. Viscosity-wise most modern oils are far below hot specs within 5,000 miles as the long chain viscosity enhancers literally get chopped up.

The oil in old cars got damaged chemically rather than wore out as a lubricant. Over rich mixtures on startup due to the primitive carburettors, poor machining tolerances meaning lots of combustion by-products making it past the rings and contaminating the oil, the increasing acidity would attack white metal bearings.

So no – modern oil is not better than olden day oil as a lubricant – far from it – it is much more chemically stable which is why it survives the extended service intervals which do reduce engine life. Notice the particle suspension agent I mentioned? They are so oil filters can be specified with MORE POROUS filter material (or they would clog with such extended service intervals) the small particles are suspended in the oil instead of being filtered out - damaging your engine – suspension is seen as a better evil than allowing them to settle creating sludge. Still think modern oil is better?

If you own your car – change your oil and filter between services – your engine will thank you for it. There's no reason why a modern engine shouldn't last 300K+ miles given frequent oil changes.