Are you changing them in the vehicle or do you have the heads off ? if they're off check the guides , if they're shot seals won't help a lot.
Stock valve seals aren't really seals they're more like oil deflectors. " umbrellas " actually. if they fit semi snug on the stem and have a diameter that covers the guide that's about all you're looking for.
If you're machined for positive seals then the choice of seals is more important. I like the viton ones they seem to work well. DO NOT use the white teflon seals unless you like smoke.