I've seen a lot of 300EX do this over time after it gets a few miles on it. All of the Hondas usually get this problem after they start getting some miles on them. It's the round black rubber seal between the valve cover and the head. It is located at the end of the cam. It's usually no big deal, but it will leak enough to get your engine oily. I would just pull the valve cover and put a thin coat of gasket sealant on it. Not much, just a thin coat. I would use Hondabond. That is great stuff and it don't cure and get rubbery like RTV sealant does. I wouldn't use RTV sealant, because the excess will squeeze out and when it cures it will be rubbery and if it breaks loose on the inside of the engine, it could stop up your oil pump or oil drain holes or something and cause a lot of damage. I learned that the hard way a long time ago. I won't use anything but Hondabond, now.