Synthetic oil is actually better than Dino oil, as it keeps your engine cleaner. The reason that your man has probably advised against it is that after running your car on Dino oil for a while, deposits build up around your seals. Switching to synthetic will clean away these deposits and often leads to oil leaks which are often (incorrectly) attributed to the usage of the synthetic oil. I have had no problems in switching to synthetic oil on well maintained engines with up to 100,000m on the clock. Although synthetic can provide extended oil change intervals, I would suggest you keep it at 5000m for the first 2 changes after the switch, as this will be when most of the crud gets purged from your engine.