[ChrisBennet]

Tell us more.
Does the motor smoke a lot when you start it up in the morning? (Rings)
If you accelerate and then let off the gas abruptly, doe you (or someone following you) see white smoke come out the tailpipe? (valve guides)
Are there drips under the car and where are they?

I've Timecerted (had machinist) some wobbly head studs so it's not unheard of. It isn't hurting anything to do them all.
Porsche was probably still selling Dilivar studs when the PO had the motor done though the mechanic should have know better than to use them.
David covered the most likely culprits but in the interest of completeness here are some others:
The last motor I did had a leak behind the oil cooler in the case. that sort of thing is rare though.
Another possibility is a leaking rocker shaft seal on the oil cooler end of the motor. Someone who didn't know enough to use non-Dilivar head studs might not have know to use rocker shaft seals.
-Chris