The normal length of trip is the decision point. Short trip (less than 8 miles, longer in winter) and the mosture/sluge formed in the cold engine oil does not get hot enought to boil off. Longer trip and the engine oil gets hot enough to boil off the moisture/sluge and the oil stays cleaner so you can go with the longer schedule. I think the schedule was created before they required synthetic oil so it is conservative, but you should consider the warranty and stick to it until out of warranty (and keep records of oil changes). I have heard stories of autos that were usually only driven short distances and the engines died with low mileage. Engines with few warm-ups like long hall trucks and farm tractors can go many more hours on oil without excessive wear. Most autos that give you oil life indication count warm ups and miles driven to calculate oil life.
