I've had good luck with a seal design that clamps onto the top of the valve guide with a flat spring and has a Teflon insert that rides on the valve stem. It's not as compact as the tiny P/C style seal that requires the top of the guide to be machined down so double or triple springs can be used. It's a good compromise between the umbrella seal that's supposed to ride up and down with the valve stem and the racing-type seals. As above, if that type of seal is used, don't use the O rings.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!