The original oil used in a vintage rain lamp was Drakeol #35 oil, but it is difficult to find today. The most commonly-used oil replacement for a rain lamp is mineral oil, which can clog the lamp overtime. Rain Lamp Fluid is commercially produced to replace the original rain lamp oil, comes in a quart bottle (32 ounces) bottle and claims not to clog the lamp. Adding oil or fluid to a rain lamp is easy to do but must be done slowly and carefully to protect the oil pump in the lamp.
Place plastic sheeting under your work area and cover it with newspaper. Keep paper towels handy in case of spills and drips.
Turn on the rain lamp.
Use a paper towel to gently wipe oil from the fishing strands where you will be working. Locate the holes in the bottom pan of the cage under the foliage and statue. There is not a specific oil fill hole. The oil will find its way through the same holes that collects oil drips from the fishing line strands. The collection basin is under the cage where the oil collects and is pumped to the top of the lamp.
Slowly pour the first pint of oil into the bottom of the cage under the foliage.
Wait five minutes for the rain effect to start and listen for a change in the sound of the pump.
Add another half pint of oil if the rain has not started or there is no change in the sound of the pump.
Wait five minutes and repeat pouring a half pint of oil at a time until the rain looks normal and the pump sounds somewhat muted.
Expect to pour at least one pint of oil but no more than three pints. Stop adding oil when the raining effect looks normal. Do not use too much oil, because it can overwhelm the pump.