I don't change the oil that often and you can buy a 1/2 quart at dollar general for $2. I find one does my tray sometimes two trays depending on which hive it is under (I have top bars). I just keep and eye on them and when the dead beetles, wax, pollen, propolis and such start to build up where there isn't enough liquid to drown them then I change them. Last year I think I changed them twice but I should have changed them 3 times here in Florida. I missed the fall change which is what got me over the winter, the hive survived but this spring I was loaded up with beetles. I crushed some beetles and changed my oil now my hives have very few. I don't know that I have ever not had any beetles but in Florida they are so prevalent.