Synthetic oil does not wear out. It gets dirt in it though. It helps the diesel turn over easier in cold winter weather. I am retired and drive fewer miles than I used to. I change oil around once a year, which is somewhere between 6,000 and 8,000 miles on each vehicle. I could go much longer.
Modern oil is far superior to the old oil. Synthetic oil carbon chains do not break down. We have been sold a bill of goods by the oil industry suggesting we should change oil every 3,000 miles. There have been some 18 wheel rigs that have made it one million miles on the same synthetic oil. They just use micro filtration and change the filters every 5,000 miles or so.