Mark,
It's usually caused by the Cam box ' rocker cover getting cold air across it and causing condensation inside. It could be water in the oil.
Look at the oil dipstick. Does the oil look ok ?
If in doubt, drain it out and examine it.
You could remove the rocker cover, clean it, and see if it recurs.
Check compressions and watch out for water level drop. If they are ok, it's probably the cold air problem.
Hope this helps.
Dave
May 2012