Many thanks for all your answers/views.

The problem occurred almost at once. The oil was introduced at room temperature and the pistol was in the same room, which suggests it should not have solidified. The action is now very rough and difficult; seems impossible to cock more than 80%.

Yes, it was neatsfoot: I bought it from a local gunsmith.

Not sure how to open the cylinder on this pistol or if I have the right tools.

It raises the question of what to introduce into the transfer port of my other vintage rifles with leather washers as I don't want the same problem. Clearly, not silicon but I wonder what is considered the best for these leather washers?

Rgds
A