The problem is keeping all the ingested dust particles getting past the air filter suspended and away from scoring polished metal surfaces, in addition to all the other work oil must do in an engine. An oil analysis can help determine how long the oil can be run, but for most non-commercial uses a set interval that is reasonable should suffice. The manufacturer's guideline usually are a a suitable point for most people.
Having said that any modern oil should be good for 5,000 km (or even 5k miles).