You say it had eased itself back to normal; are you saying then that it had physically stopped turning? I don't think the viscosity of the oil is the problem here.
If the lubricator was of the oscillating type (not sure of the exact name, but the type LBSC used to specify) then the cylinder would have just lifted off its seat if it was blocked; it would have still kept turning. If it is of a less forgiving kind then a physical blockage or stuck valve may have caused it to jam solid, but I would suspect its the ratchet or clutch giving you the issues here. Failing that, a mechanical issue with the pump itself. But in my opinion (and I'm still very much an inexperience amateur) I don't think the viscosity of the oil would cause the issue you are describing.
I suspect the pump body itself had something in it which caused it to jam.
Edited By Jon Lawes on 05/04/2021 19:17:36