If you were using an oil sampling service, you could use either type of oil, and go untill you hit a threshold limit, usually determined by you and your sample report provider after a test run of samples and intervals. This could be way past what you would ordinarily expect, or, if you have an older engine (any component) that isn't doing well, it might be somewhat less.
If you run synthetic oils, you have to sample in my book. The synthetics are too expensive to use unless you sample and get the maximum life out of them, again at a point to be determined by you and your sample service.
But sampling is the only way to get the maximum life out of any type oil, simply changing at a specified interval is convenient, but you may be wasting oil and spending too much, or you may not be changing often enough. Without sample reports, who knows?
The same reasoning goes with filters. Premium filters may do a good job, but how do you know without sample reports?
I've gone both ways, and using sample reports to spot trends and problems is a very valuable tool. But I've changed thousands of gallons of oil by the number of days, weeks, months or hours, also by the gallons of fuel consumed by an individual machine. Using the last method can mean that even two identical machines can have different change periods because of the variables in the way they are used and the operator work methods. It is just about the most accurate way of changing oils short of simply going by the sample reports.
Too many people forget that there is oil in their transmissions and differentials, and some other compartments. They never check them till a failure occurs, they never change the oil, till a failure occurs, they never sample. Sampling these fluids a couple of times a year won't break you, but a sudden component failure just might.
Current engine oils are exceptionally good as are filters. I like the Texaco Ursa and Shell Rotella lubricants. But I've used Dryden (now Castrol), Kendall, Mobil, Gulf (Canada), Sunoco, and many others with good success. Wix filters are my favorite, available in many sub-brands like NAPA, Car Quest, Big A, etc,. I have used the Baldwin's, Fram, Purolator, Caterpillar, AC Delco, and many others with little problem.
You can play a bit with filters -- such as using one that has a smaller micron rating or perhaps one that has additional oil capacity, but you have to be careful and give complete regard to the manufacturers specs and directions. There may be a special reason why the maker does not use a certain filter, even tho it looks to be superior by the numbers. So beware.
I once changed engine oils for an outfit that insisted on doing it every one thousand miles, in their case, about once a week. That oil came out of their Cummins 400's cleaner than when it went in. Just about the only diesels I ever saw where the oil was not black with soot. On the other hand, I've seen some where it was changed once a year, whether it needed it or not. In neither case did the owners have a clue as to what shape their oil and their engines were in.
Generally speaking, I've not used much synthetic oil, mostly because of the cost, but also because it doesn't make sense to run it without sampling. I also have just not had a real need to use it as regular oil blends work nearly as well in most of the iron I've been around. Now some component manufacturers are using synthetics as factory fill for tranny's and diff's with a recommended change interval of one million miles. That's a long time and unless they are going to guarantee a full cost replacement warranty on those components regardless of miles, I'm going to check and sample for my own protection, as the best warranty in the world does not pay for downtime and breakdowns or road service in most cases.
One last note and it's off to bed for me. If you use a standard diesel engine oil and have a minor problem, say a small leak in the engine oil cooler. That cheaper oil may not protect your bearings or other parts and you could be faced with a mighty expensive O/haul. A premium oil may actually protect an engine that has swallowed an entire radiator's worth of coolant and kept on running till it overheated. I've experienced a couple of these and after fixing the problem (busted oil coolers), flushing the engine properly with a fine solvent and fresh light weight oil, then filling with the regular premium oil, found that no further problems were to be had and the engines ran just fine and showed clean on the next sample. If we had been running a lesser oil, I don't think we would have gotten off so easy.
Good luck!
