My previous Vauxhall (MY 2001) had 20k service interval, and my Renault (MY 2005) 18k, albeit 'or one year' in both cases. And both officially run on semi-synth (i.e. not even full-synth).
I guess it's based on the assumption that cars that do 20k a year will do most of the mileage on Motorways.
But again, it's more to do with tempting fleet buyers. And, it would be foolish of a car manufacturer to stipulate oil change every 6 months if the only benefit is that the engine will make it to 300k rather than just 200k - very few new car buyers will actually care about that.
But there's a bigger issue here - maintenance scheduled vary considerably between manufacturers, in spite of the technical similarity of the affected products. This clearly demonstrates that service schedule are a 'guesstimate' at best.
MB stipulates brake fluid change every 2 years, Renault every 3. I doubt that Renault have something in their break fluid or brake system that makes their brake fluid last longer, especially as the recommendation by the manufacturers are (with few exceptions) not model-specific but across the board.
Another example is spark plugs - MB say every 4 years, Toyota say every 62k miles (presumably translated from 100k KM) regardless of age, and Renault say they are good for life. Yet - with few exceptions, e.g. Iridium or quad-electrode plugs - the technology is pretty much the same. It seems that MB's consideration of preventing the spark plug thread from corroding is not shared by other manufacturers - and they all have alloy heads as well.
Or ATF - some boxes are 'sealed for life' (Vauxahll, Renault, and BMW), while other should be changed according to schdule - is there really such a big difference in the design of the autobox or the make of the ATF?
I used to do some work for a factory that made filters (fuel and oil) for manufacturers, i.e. Tier-1 supplier. The filters are all made from the same paper element, and after coming off the production line are painted in the respective manufacturer's colours and logo (e.g. Vauxhall, VW, etc). Yet again once these filters make their way into the cars the replacement schedules vary considerably.
Another issue is to do with the use of mileage - is that, as anyone working with diggers and earth mover will know, engines care more about working hours than mileage covered.
So in short a '20k miles or 12 months' statement is at best an educated guess by the manufacturer. If you know no better, then I would suggest to stick to the manufacturer's (very broad) recommendations. With regards new cars, sticking to manufacturer's schedule will also help keep its value come sale time, which is a good thing by itself, even if you as the first owner may never know the difference or directly benefit from adhering to strict maintenance regime.
)
