I would go with your plan. Engine heat up after slowing down is normal, with teh fans kicking in at 100C or so.
The only other source of oil would be from the oil cooler at the filter housing:
http://www.peachparts.com/shopforum/mercedes-benz-performance-paddock/33832-m104-engine-oil-cooler-retrofit-e320-c36-etc.html