I often hear folks ask what to do about a short trip vehicle and the typical advice is to take it on a long freeway run periodically. So that does not really help?
It really depends now on lot on each specific case. If you accumulate water in your oil (creamy white sludge on oil filler cap) regularly, I'd change the oil every 6 months. To use a synthetic oil is a waste of money in this case. Even the best oil can't withstand these conditions for long. I have seen a case with 80% city cycle driving and 20% autobahn driving (both in km) in total 6.500 km with a longlife oil - the oil analysis was a desaster. It looked like as if it was driven for 50.000 km high speed driving.
If you do not see "white sludge" in your engine, the oil ages most of the time in a "normal way". In this case you may increase your service interval with a synthetic oil compared to a dino oil.