Oil seals are fitted to the valve stems or the guides on both intake and exhaust valves. They prevent too much oil passing down into the combustion chamber.
The coil spring on the outside holds the sealing edge against the valve stem. The angle at the top of the seal forms a small reservoir of oil to lubricate the stem and guide. If there is too much oil there, carbon deposits form in the port and on the valve head.
Umbrella seals shed the oil and keep it away from the end of the valve guide. Worn seals or guides or too much valve-guide clearance will let oil pass the intake valve.
The inlet valve is more likely to pass oil through its guide than the exhaust valve. This is because of the low pressure in the inlet port that draws in the oil.
The exhaust valve can still have problems because of exhaust pulsing. This creates a low pressure area behind the gases, which can cause oil to pass down the valve guide.
Some engines however don’t use oil seals on their exhaust valves.
Valves are normally held on their seats by 1 or 2 coil springs that are compressed between the cylinder head and a retainer on the valve stem.
The spring retainer is held on the end of the valve stem by conical shaped collets. Collets are also known as cotters, keepers or keys. The springs usually have their coils closer at the bottom than the top. This makes different parts of the spring vibrate at different frequencies, and prevents wasteful valve spring vibration. They can also be made of wire with an especially shaped strong section that limits valve bounce.
Unless the valve is held on its seat, it also allows leakage from the combustion chamber. Carbon builds up on the valve stem.