Amount of oil poured into the cylinder have to be accurate does not have to be accurate. A couple pumps of an oil can, or a capful of oil -- but make sure all four spark plugs are out. See Engine compression test

The valve stem seals go bad due to:
* Heat causes them to get brittle and break
* Heat causes them to get hard and so stop sealing well
* Tearing or other physical damage sometimes occurs
The don't dry out as they are surrounded by oil.

You can replace the valve stem seals without removing the cylinder head. It's far more work to remove the head.