No the diesel pump is not lubricated by oil on the pneumatic governor equipped pumps, it is lubricated by diesel that is bled down from the pumping elements.
On the mechanical governed pump on the later Dexta and Super Dexta, the Minimec, oil is used to lubricate the governor only, the pumping elements and cam box are still lubricated by diesel.
At least that was correct when I was a Simms trained engineer back in the day and working on these tractors.
Yes I would suspect you have a leaking lift pump diaphragm. That is the usual cause of excess diesel in the cam box.