Many cargo expediter drivers run synthetic oil over 15,000 miles, but they do not post in here. There are expediters that run from Laredo tx to Michigan many times a month. Long hours of idle are normal, some as long as 4 or 5 days steady, year around. In four years , 300 to 400 thousand miles is routine. Most are running standard v8 engines in cargo vans. Most folks in here do not get anywhere near this type of hard running--no Sunday school driving in expediting, it is usually u know whats to the wall boys. You cannot do the 3000 mile oil change out there on the road, that 3000 miles can go in 7 days, and you can see how expensive that gets real quick ,plus it becomes a nuisance, but some do it. Very very few are big on oil additives and such.
So yes, the synthetic users run it a lot longer then conventional, it just makes sense to do it.