Which oil you use is for some reason something people are willing to go to war over, don't understand it. I know the US Army uses Mil-L-7808 and Mil-L-23699 in their turboshaft engines. Both oils are synthetic.
Knowing the thousands of gallons of oil used annually by the Military the cost accountants would have them go to a "cheaper" oil if the performance benefits of synthetics were worth less than the additional cost of the oil.
The Military changes the oil when it tests bad, not sooner. They take oil samples like every 5, 10, or 25 hours dependent on what the engineers had decided. They monitor the stuff in the oil and can lead to early removal of the engine due to a failing bearing, etc.
We as owner's of vehicles do not test our oil, we just change it when our change out interval is accumilated on the oil.
There are superior features of synthetic which pay benefits under severe operating conditions. I use synthetic in both of my vehicles - 88 XJ Pioneer and a 2003 Buick Le Sabre. I switched the XJ at 133,000 miles but before I did I had changed the oil pan, valve cover and oil filter mount gaskets so most of the seals were new.
I would recommend changing the oil at the interval in your owner's manual, use severe or normal schedule depending on how you operate the vehicle. If you are doing oil samples then you can extend the oil service life until it is fully "dirty".
With that said, it up to you what oil you use - normal or synthetic. The reason we change oil it to remove the bad stuff from engine wear so it does not cause accelerated wear.
One last thing, people over look the Oil Filter but it is just as important. If the filter goes into bypass then you get no filtering. Back in the early 80s when the talked about Mobil 1 being 25,000 mile oil the thing not mentioned was the life of an oil filter. The Engineer who talked to my Society of Automotive Engineers (SAE) chapture in college on this issue said to expect 10,000 miles out of a filter. If you run your oil longer it was recommended to change the filter every 10K miles even if you kept the synthetic in for 25K miles.