Let's be clear on this, o-rings and umbrella seals are not seals, they are oil deflectors. The only true valve stem seal is a positive type with spring wire holding them tight to the guide with a semi tight fitting lip seal.
The o-rings keep the oil that runs down through the valve keepers off the stems. Most valve keepers butt right up to the o-rings.
The umbrella seal does just like an umbrella does, it deflects the oil ( or rain ) out away from the stem but doesn't actually seal to anything. Oil mist and splash still reaches the stem. If the umbrella seal sticks to the stem and rides up and down with the valve, they can cause more oil to enter the guide because the umbrella is catching and funneling the oil right to the guide.
A positive seal meters the amount of oil to the stem as long as it's in good shape, but like all seals in an engine, they loosen up with age.
The stem to guide clearance is way more important than the seal type.
