meh... use cheap oil for now until you find the source of the problem. Is so much gas getting into the crankcase that you see a noticeable difference of oil level height in the sight glass?
this is strange because your fuel petcock should only be flowing fuel when vacuum is being applied to it from that small vacuum hose on the back of it when you have the petcock in the "on" position.
your main problem sounds like one of the needle float valves is either stuck or not seating well due to debris.
if this is your problem, you normally need to remove the carbs and clean out the debris that is holding that valve open. You can try tapping the side of the carb bowls with the plastic end of a screwdriver to help dislodge whatever dirt that might be the problem. If you're lucky (as your user name implies
), you'll be able to fix it using this method.
At the very least, drain your carb bowls and catch and examine the gas for any signs of dirt in any of the carbs.
http://faq.ninja250.org/wiki/Draining_the_carbs