Valve Stem Seals
Different manufacturers use different types of valve seals on their cylinder heads. Aluminum heads typically have positive lock valve seals. These are the preferred seal these days. They're installed onto the cylinder head before the valve, and an installation tool is required. These seals snap into place onto a machined groove in the head. Check the manual; intake and exhaust valves are typically different colors and sizes.
Another popular seal is the umbrella type valve seal. These valve seals used to be everywhere on all kinds of engines. Umbrella seals are installed on the cylinder head after the valve's inserted into its guide. Polish the valve with crocus cloth and lubricate it with oil or assembly lube before installation.
O-ring seals are located on the valve stem in the third groove down, under the two retainer grooves. They're installed after compressing the valve spring. It's important to lubricate the seal with oil before sliding the O-ring over the valve stem.
Valve Seal Symptoms: When valve seals leak, oil enters the combustion chamber causing the vehicle to emit blue-gray exhaust from its tailpipe. The exhaust smoke is most apparent at start-up after the vehicle's been sitting for a while. It's because the oil on top of the cylinder head has had time to seep into the chamber. Once the engine has run, and the oil has cleared out, the smoke may disappear or not be so apparent until next start.