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the chain guides are plastic bits that put tension on the chains in the engine. the purpose of the chains is to allow the turning crankshaft to drive the camshafts. these plastic pads will wear. one symptom is plastic bits in the oil. the other symptom is that the cams have sensors that tell how much they have gone out of spec - this is called 'cam deviation' and a shop with the proper diagnostic tools (durametric, pst2, piwis - all types of advanced computer code readers) can determine the deviation values. anything under '6' is ok. anything over 6 is considered out of spec by Porsche, but the 'check engine light' won't come on in your car until deviation hits 9 or so.
all of us experience chain pad wear. get your shop to tell you your deviation values. if under nine (ie, no engine light on) keep driving, and change your oil MUCH more frequently (the oil is what keeps the pads lubricated and reduces wear).
if you are going to do the repair, ask your mechanic if they could do the job with the engine in the car - this could save you up to half the labour cost. also, it's not a 'complete' engine teardown, just removal of the cams and cam covers (ie, the heads don't come off) so get them to itemise the work they intend to do and bring it back to us - we can tell you if they are playing you or not ...
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