In my experience automatic chain oilers are a poor investment. The problem is that when you have fresh oil dripped onto a chain, then go out harvesting (whether in a combine or a baler) the dust from the feild sticks to the oil and gets carried into the rollers. This actually wears the chain muh faster. Personally I use Moly coat chain lube at the end of the day, that way the lube has soaked in and dried up by morning. It is a little bit of a pain having to lube some chains, but I find they last longer that way.
