ah that thing.. its part of the positive crankcase ventilation! your bloody right it is a pressure valve!! but it doesnt say it controls oil pressure! but i've never had a mk 3 gti got a vr6 though..

so i havent had to study the engine...
according to my haynes manual heres what it says!
disconect the crankcase breather hoses from the pressure regulator valve, mounted on top of the camsahft cover. if crimped-type hose clips are used, cut them off and replace them with standard worm-drive clips on refitting. slacken and withdraw the retaining screws and remove the regulator valve!
thats all it says...
i hope this helps....