Loving the Cayenne!
AO Silver Member
Join Date: Apr 2009
Location: United Kingdom
Yes, to replace either seal, the gearbox needs to come off.
Clutches tend to develop a judder, when the friction surface becomes contaminated with oil. The synthetic oil, that you have just installed, will most likely be thinner than the previous oil, and therefore more likely to seep through the crankshaft oil seal. (at the end of the engine, right next to the clutch.)
Going back to the 10/40 oil may cure the problem temporarily, but, if the CS oilseal is the problem, then it must be in need of replacement anyway.