Back in the late eighties I was at the dealer and there was a 190E 2.3 in the service lane that had a ticking engine. My service advisor told me that the owner had never changed the oil, and the car had about 25K miles. Of course, it was still on warranty, but MBZ would have been justified for not honoring it because the owner did not observe the 7500 miles or six month oil change requirement.

Most new cars that require synthetic oil have oil quality monitoring systems that tell you when it is time to change oil. If you have an older car and want to use synthetic, I would not go beyond 10K miles or one year, which renders synthetic not cost effective, because it is at least double the price of conventional oil.

Back when I was driving my 190E 2.6 every day I changed the oil and filter every 5K miles, which worked out to about three times per year.

Since none of my cars accumulate much mileage now I change the oil and filter every year - use conventional spark ignition engine oil (API service category SL) on modern cars with catalytic converters and HD diesel engine oil (API service category CI-4) on the vintage cars without converters. CI-4 oil has a richer additive package that is good for the vintage cars with carburetors that tend to suffer more oil dilution due to imprecise mixture control, but the combustion byproducts of these additives are not kind to converters. Also, modern fuel injected emission controlled engines have much more precise fuel metering, so they don't necessarily need the additional additive concentration of the CI-4 oils.

Bottom line - unless you're an engineer with plenty of experience in engine lubrication requirements, follow the manufacturer's recommendation for oil change interval and service products.

Duke