I am not sure if the older Vikings used the same grease that the 70's and 80's ones do, but if so, that grease hardens from lack of use after about 6 mos...
I was told that the only solution IN THAT CASE would be to open it up, remove all of the grease and redo it.
However, if the machine just uses oil, and the oil dries up, the trick used in the early days was to have two oil cans, one kerosene, one oil. When the machine gets "gummy" you oil it with the kerosene, run it lightly for a bit, give it a good wipe down, then oil with the usual oil. The Kerosene will breakdown the varnish that oil dies into.