We have a couple of glass oil lamps that were in daily use in my grandfather's home, and have had oil in them (with refills) since at least the 1950's or before. Not ever a problem. The key is to be to turn the wick down when not in use so only a little bit shows above the burner assembly that holds the wick. This prevents/reduces any evaporation (not much, if any, in my experience) but also stops the build up of dust/dirt etc. caused by airborne dirt landing on the exposed wick and sticking to the oil and wick. When you want to light it, just turn the wick up and light. This automatically makes sure there is no sticking. Stored this way, I have never noticed a problem. These lamps get used 1-2 times a year when our electricity goes out for a day or two at a time.