Cam covers can leak even if the breathers are clean - they go hard and just start to leak.
This happened with mine.
To this one - plug leads or wrong plug lead order
But surely that occurs only when :
a) inferoir gaskets are used, thus leading too:
b) overtightening of the cam cover to compensate?
People and garage gorillas seem to forget that it's only a little plastic cover, and it's the rubber o- rings and gasket that actually do all the work, but they still wind it up for all it's worth causing it to warp, which over time causes it to fatigue.
As for the problem here, I remember when I chaged mine, I too had oil in the plug wells, and tried to get most of it out, but some invariably fell into the bores.
As Mark DTM says, a mile or two of running sorted it. What I did find though which is confirmed all that has been said already, was that the plug caps themselves had rotted through.
Invariably this not only causes a weak spark, but has also been known (check forums old posts) to knacker the DIS pack.
If you haven't already done so, then clean the breathers, but also check and if necessary replace your leads as a first port of call.
Don't look for cats and things just yet, - that's expensive troubleshoot the easy stuff first
