I don’t work for a motor oil company, but I do work for a company that makes the additives that motor oil companies put into their oil (specifically, I make the detergent).
Not all motor oils have the same certifications. In the US, the SAE sunburst is the most common one you’ll see, but there are other certifications. In particular, some European automakers have their own, preferred certifications. If you’ve got an unusual car, you’ll want to look for the certification your carmaker recommends.
That said, two motor oils from different companies but with identical certifications are essentially interchangable. The specific chemicals that’re being added to do the job may be slightly different, but there’s not enough profit margin to add any more than is strictly required to pass the engine tests for certification.
Mind you, those engine tests are notoriously fickle, and everyone in the industry argues if they really predict actual performance. But they do ensure that most motor oils are very similar.
What you want to look out for are premium motor oils that make un-certified claims of performance. In particular, motor oils for “longer-life” cars. These do have extra additives in them, but not enough extra to justify the premium that you’re paying.
Beyond that, I use synthetic motor oil, but that’s more of a personal prejudice against the naturals than insider knowledge.
As for change intervals, you basically want to change your motor oil when its basicity is used up (in neutralizing the acids generated by oil degradation). The overbased detergent is usually the component that’s going to be used up first. Unfortunately, unless you have a fleet of expensive trucks, its just not cost-effective to keep testing your oil – its easier to change it at the recommended mileage, which is determined by average driving, with a generous margin on the “change early” side.