i did this:
http://faq.ninja250.org/wiki/How_do_...Fork_oil_level
went from 205mm level (of air) to 160mm. Very happy with it.
(i did after a while i went with the "correct" way of swapping the stock springs out with a ninja 500 springs, and then later the SonicSpring.)
now, i am not sure what's the oil/air level on the new 300. so if it started off high in oil from the factory already then no i don't recommend going the "add oil" route.