Opinions vary. For YEARS engines never had seals on the exhaust. Look at the early Hemi for example. There is ZERO NEED for seals on the exhaust, because the exhaust valve stems point DOWN. Oil ain't defyin gravity. The truth of the matter is, if the guides and stems are in proper shape with the correct clearance, there damn near ain't a need for seals on either side. Why on earth would oil be pulled into the intake guides when there's a HUGE intake port? Honestly, about the only time one burns any appreciable amount of oil through the guides is when the guides are worn slam out. Even the older Chevy small blocks used ONLY a small o ring to seal the valves and they WORKED FINE. It's long been my opinion you can seal the valve stems too much and starve the guides.
Why on earth would oil be pulled into the intake guides when there's a HUGE intake port?
Rusty, you and I agree on a lot of things, but not this. The vacuum in a passage is effective in all of the passage, not just its largest cross sectional area. To prove that, get your wife's vacuum cleaner and a sharp pin and poke a tiny hole in the hose, tell her not to worry as such a small hole will not result in a vacuum leak,,, find out how that works for you. The vacuum or actually negative pressure in an intake port will suck the oil down and off an intake valve stem. And the 1/2 thou a side clearance in a valve stem is plenty of room for hot engine oil to run through. And it's not just an oil consumption issue, the oil that gets down the valve stem can cause coke to build up on the back of the valve, that hurts performance
Concerning exhaust valves, the pressure is blowing out so one may ask why put positive valve seals there. Because the pressure from the exhaust wave will also blow through that 1/2 thou a side clearance taking the lubricating oil out and leaving carbon and soot and acids from the combustion process in the valve guild passage.
Will the engine run without positive valve seals, sure it will, but it will not run as well over the potential life span of the engine. There are several reasons why today's engines last much longer than engines that were around when we first started driving. Better valve seals is one of them.
Back to Pista's question: Umbrellas seals work like the name suggests, they deflect the oil splashing around under the valve cover away from the valve stem like an umbrella. They deflect enough to be effective but are not as effective as positive valve seals.
Positive valve seals are generally sold by valve guide outside diameter and by valve stem diameter. There is a little bit of interference fit on the guide to make the positive valve seals stay in place.
I suggest measure to get the valve stem OD and the guild OD then shop Summit or some generic valve train supplier like Comp to find the positive stems that fit your application. In some cases the OD of the valve guide is turned down to come up with a readily available positive seal that fits your application.
When installing valves through positive valve seals, some valve seal kits include a thin vinyl sheath that fits over the valve lock grooves to protect the seal during the install process. If the kit you get does not include that, stretch a single wrap of vinyl electrical tape over the locks and lube with oil prior to install.
