The gentleman that repairs and services my machines for the last eight years uses Vaseline for the gears. I was surprised and asked him about it and he said that the Vaseline doesn't harden like some grease does.
It did reminded me of my old Viking and my mom's 1950's Necchi, which both had a problem (at different times) and it was recommended by different repairmen that the parts had to be cleaned to get rid of the gunk from the grease that hardened on the gears. It wasn't a cheap repair for either of the machines.