I| had a problem with stiff tuners on my mandolin, on the G strings. I applied tiny drops of oil with the tip of a matchstick to the cogs - just three or four drops - and then wound them round and round (and round) to spread the oil around. I have been warned not to get oil anywhere near the wood. They are Schaller machines. This was about 20 years ago, and they've been no trouble ever since. Otherwise, I can recommend Schallers as I have 3 guitars, including a 12 string, with them on. Other brands are reputedly just as good, but I would avoid cheaper ones.
Richard