I've been reading different posts about which type of seal is better, umbrella or o ring. Can someone explain how each one works? Does the umbrella just got over the valve guide or in the groove where the o ring would go? If I use the o ring type does the retainer slide up and down over the o ring? The ones in the head gasket set are bigger than valve stem is that normal? I haven't dug this deep in an engine before. Thanks for reading.