I've just replaced the valve body on the motorised valve serving the upstairs zone in my house. It was extremely stiff to turn even with the head off and a pair of pliers on the shaft.
It wasn't worth the aggravation of trying to remove the spindle to investigate and possibly clean and lubricate with a risk the shaft seal would leak afterwards.
Fortunately I had installed gate valves either side in the pipework when I first put the system in so it was a few minutes work and no drain down required.