Al3x,
How could I tell the Hein Gericke oil was past it?
The key indicator was that it stank like old overheated (toasted in the engine) very high mileage oil. In my teens I worked in an auto garage and we occassionaly saw and smelt this when some one had missed a number of oil changes. The Hein Gericke oil in my bike got into this state in about 1,500 / 2,000 miles. This is the usual mileage before I change the oil.
I usually use Motul / Silkolene-Fuchs / Motorex in the Superduke, never had this problem before, never had it since, riding style etc has not changed before, during or after.