My oil rep told me that if a car has say 80k on it...all on dino, and you fill it up with synthetic to have a shorter OCI on your first run, as most synthetics clean better, and therefore you could be running with really dirty oil if you go for a long time. He recommended like a 3-5k change, rather than the 10k or so when normally using synthetic. After that first short interval, you should be fine for going longer. Granted this is from a guy who sells oil for a living...