This article is from the VW Technical FAQ, by with numerous contributions by Jan Vandenbrande others.
A: All/most? VWs made after 1985 have hydraulic valves, which adjust
themselves. Cars before that need periodic adjustments.
Check for the recommended intervals in your manual, do more frequently on
older cars & with periodic compression checks. Badly adjusted valves reduce
power, increase pollution and may lead to burned valves (valves cool thru
contact with the head).
It's a fairly easy job to do, you DO need two special tools: One to
compress the valves and one to remove the adjusting shims. They can be had
at most car specialty stores.
All VW engines based on the old 1500 thru 1800 block use calibrated metal
shims (tappets) the size of Alka Seltzer pills for adjusting the valves.
You first measure your clearance, then determine how much off it is, take
out the old shim, calculate what new size you need, and race of to your
parts supplier (or use a shim from another valve).
 
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