When they are worn, as in the problem I had with my '73 when I bought it, you will get a leak down when the car is sitting. When you start it up you get smoke and then it clears up and runs without smoking.
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
R & D To many easter eggs!
So I was bored today....
The STOCK O ring( Valve seal) fits below the HAT.
I have to assume the following
( trying this with thin oil)
The valve locks are not a perfect seal.(DUH)
When the engine is running the oil trickles down the rocker arm tip and on to the head of the valve for lubrication, the oil pools up on the top of the valve ( as best as possible with it running) but due to motion.. it flies all over the place ( not a bad thing).
When you stop.. the leftover oil now drips down to ther top of the valve( depending where the engine/cam stop) and if the seal is dead/deformed/missing the leaking oil wick's down the valve, past the guide and leaves a pool of oil sitting on the top of the valve. Vroom...puff of smoke and it clears.
A lot of factors combine during this process, how worn the guide is, how bad the
( what's left of the) seal is etc.
So I have sold myself on umbrella seals.
The factory ones are not that great and there is a good chance they will not seat properly when your re installing em.
I would go with the umbrella seals. This is best way to go, some people only use them on the exhaust side but I used them on both when I was bulding a big block for my Chevelle. Never had any problems with them.
I have the same problem with my 74. I have never heard of the seals doing any real damage by leaking so am gonna leave them until i get the crate engine I want and then rebuild the stock engine myself. You know it's getting oil, mine smokes on the passnger side for maybe two minutes after sitting for a day or two.