400 oil leak

[Hydroglen]

Hello,
My 400 has always had an oil leak /seepage coming from the head. This is seen as oil around the head near the head bolts and spark plugs. It oozes out of the left side of the head.
I have cleaned the head many times but I am at a loss as to were it is coming from. After cleaning I have tried running the engine wilst observing and some 15 minutes later nothing.
It is not enough to see a steady leak.
If I go for a drive of say an hour, then I will find wetness and some streakes out the left side.
The rocker covers are not leaking. The cross tubes are not leaking.
I could perch myself on the left fender and have my wife drive the car wilst I observe. Before I go that route, I was wondering if anybody else had some thoughts on the subject.
Thanks
Dorien