It will be difficult to tell you how to fix the oil leak if you don't know where it is coming from. Most oil leaks do only become visible after a period of time, such as in your case, 5 minutes or so. This is because after a period of time, the oil gets hotter, which makes it thinner & easier to find places to leak. The best thing that I can tell you do to is to first, check your oil level often, be sure you do not run it low on oil. Next is try to get the bike up on a lift & take some carb cleaner & thoroughly clean the motor. Now place a clean piece of cardboard underneath the motor, crank the bike up & let it sit there & run until you start to see some oil drops on the cardboard. This will make it much easier to find your leak.
Hope this helps