Let's talk $$$. M1 oil change with filter costs me about $50 plus my time. A four pack of oil samples was about $70 last time I bought one. If I can have confidence how the oil is performing in my engine under the conditions that I drive, and I can go to extended drain intervals, I can easily save far more that the $70 cost of the samples and the few dollars for postage to send the samples in.

If it gives you peace of mind, that is the important thing.

I change the oil and filter on my 300D (and other cars) once a year whether it needs it or not. But then none of them get driven more than about 6k miles per year.

With today's superior oils, I do not see any reason to at least run the vehicle to MB's recommended mileage/time interval. For the '95 E300, I'm going to 7500 miles or one year....whichever comes first.

We never talk duty cycles. A lot of highway use versus say ninety five percent of short in town trips. Where contaminates in the oil build up much faster.

Oils do not now or in the past basically wear out I believe. In diesels especially the soot component that accumulates in it is perhaps the most determining factor of practical usage. These short trip cars usually have moisture contamination from condensation also present plus bad acids can develop from this.

People have to just use their own judgement of what they should do. Other than manufactures recommendations or oil testing if going outside the conventional recommended envelope.

Almost any car I can think of should get a fairly frequent run getting the oil hot enough to boil of the condensate at least.