TL;DR
Wash an oil dispenser with water as hot as the tap runs plus a degreasing dish soap, using a bottle brush or a tablespoon of coarse salt swirled hard as an agitator. Then dry it completely -- water left inside is what turns the next fill rancid faster, and a bottle that is not bone dry should not be refilled.
Why oil bottles get a sticky brown film
Cooking oil oxidizes when it meets air, light, and warmth. That reaction thickens the oil into a tacky varnish that clings to glass and plastic, and it smells stale and slightly waxy.
Every pour leaves a thin coat on the inside wall. That coat is exposed to far more air than the oil in the body of the bottle, so it goes rancid first and it goes rancid fast.
Topping up a bottle without washing it puts fresh oil on top of old, oxidized film. The stale flavor transfers, and it is the main reason a refillable dispenser starts producing oil that tastes worse than the bottle it came from.
The same oxidized film shows up on every oily kitchen tool, which is why degreasing rather than scrubbing is the theme throughout the guide to cleaning kitchen tools without damaging them.
How do you clean an oil dispenser bottle?
Pour out the old oil, then wash with water as hot as the tap runs and a degreasing dish soap. Add a tablespoon of coarse salt and swirl hard, or use a bottle brush, to scrub the inside wall. Rinse with hot water, wash the spout separately, and air dry completely before refilling.
The full method
1. Empty and drain it. Pour old oil into a sealed container for disposal in the trash. Never pour cooking oil down a drain, where it hardens and blocks the line, and never into a yard waste bin.
2. Wash with very hot water and degreasing soap. Heat is what thins the film. Fill the bottle a third full, add soap, cap it or cover the opening, and shake hard for thirty seconds.
3. Add a scrubbing agent. A tablespoon of coarse salt or a handful of uncooked rice, swirled vigorously with the hot soapy water, scours the wall where a brush cannot reach in a narrow neck. A long handled bottle brush is better if the neck admits one.
4. Repeat if the film persists. A badly neglected bottle usually needs two or three rounds. That is normal and it is faster than scrubbing harder.
5. Wash the spout separately. This is where most people stop early. Take the pour spout, cap, or pump apart and wash every piece in hot soapy water with a small brush.
6. Rinse with hot water. Warm rinse water leaves soap behind on an oily surface. Hot water carries it away.
7. Dry completely, upside down. A drying rack, a bottle stand, or a rolled paper towel wick. Give it several hours, or overnight.
Why drying matters more than washing
Water and oil do not mix, and water left in a bottle sits underneath the new oil as droplets.
Those droplets accelerate the breakdown of the oil, and they create a wet spot where spoilage organisms can grow in a way they cannot in pure oil. A dispenser refilled while still damp goes off noticeably faster.
There is no shortcut. Air drying a narrow necked bottle genuinely takes hours. Plan the wash for an evening and refill in the morning.
Storage containers with seals have the same rule, since a wet gasket groove is a problem on its own -- see how to clean jar lids and rubber gaskets.
Cleaning the pour spout
A pour spout is a small tube, sometimes with a hinged flap or a ball valve, and it holds the oiliest residue in the whole assembly.
Take it off. Almost all of them pull or unscrew free.
Soak the parts in hot soapy water for ten minutes, then run a small brush, a pipe cleaner, or a straw brush through the tube. Work the hinge back and forth under hot water so soapy water flushes the pivot.
If the spout has a rubber or silicone seal, remove it and wash both sides. Oil creeps under seals and turns sticky there.
Some spouts are simply past saving. Replacements are cheap, and a spout that still smells stale after two washes should be replaced rather than reused.
Sprayers, misters, and pump dispensers
An oil mister has a pump, a dip tube, and a fine nozzle, and it clogs when oil oxidizes inside the narrow passages.
Disassemble as far as the design permits. Wash the reservoir the same way as a bottle. Soak the pump assembly in hot soapy water and pump hot soapy water through it, then pump clean hot water through until it runs clear.
Dry every part fully before reassembly, and do not fill a mister with anything thicker than a light oil unless the manufacturer says it handles it. Thick oils clog the nozzle almost immediately.
What to avoid
Cold water. It does nothing to oil film. Hot water is the whole strategy.
The dishwasher for a narrow necked bottle. Spray arms cannot reach inside, so the interior comes out exactly as oily as it went in, plus wet.
Abrasive pads inside glass. They scratch, and scratches hold film more tightly.
Bleach. It does not cut grease, and it must never be combined with ammonia, with vinegar or any other acid, or with rubbing alcohol, because each combination releases a toxic gas.
Refilling without washing. Old oxidized film contaminates the new oil immediately.
Storing oil next to the stove. Heat and light are what drive the oxidation in the first place. A cool cupboard doubles the useful life of a bottle.
Measuring tools pick up the same oily residue and respond to the same treatment, covered in how to clean measuring cups and spoons that stay sticky, and so do brush bristles, in how to clean a pastry or basting brush.
Cleaning a dispenser reduces stale flavor transfer and residue. It is not a substitute for buying oil in quantities you will actually use.
Source: USDA Food Safety and Inspection Service guidance on food storage and rancidity
FAQ
How do you get rancid oil smell out of a bottle?
Wash with water as hot as the tap runs and a degreasing dish soap, adding a tablespoon of coarse salt and shaking hard to scour the inside wall. Repeat two or three times if needed. If the stale smell survives several hot washes, replace the bottle.
Can you put an oil dispenser in the dishwasher?
Not usefully if it has a narrow neck, because the spray arms never reach the interior and the oil film stays. Wash bottles by hand with hot water and degreasing soap. Small removable spout parts can go in a dishwasher basket if they are labeled dishwasher safe.
How often should you clean an oil bottle?
Every time you refill it, without exception. Fresh oil poured onto old oxidized film picks up the stale flavor immediately, which is why a refillable dispenser often tastes worse than the bottle the oil came from.
Why does my oil go bad quickly in a dispenser?
Air, light, warmth, and leftover water are the four causes. A clear bottle stored near the stove is the worst case. Wash and dry it completely between fills, keep it in a cool dark cupboard, and fill only as much as you will use within a few weeks.
How do you clean an oil sprayer or mister nozzle?
Take it apart as far as it allows, wash the reservoir with hot soapy water, then pump hot soapy water through the pump assembly followed by clean hot water until it runs clear. Dry every part fully before reassembly, and use only thin oils in a mister.
What should you do with old cooking oil?
Let it cool, pour it into a sealed non recyclable container, and put it in the trash, or take larger quantities to a local collection point that accepts cooking oil. Never pour it down a drain, where it congeals and blocks the line, and never into yard waste.
Bottles and spouts are quick. The kitchen surfaces the oil actually lands on -- the range, the backsplash, the cabinet fronts -- take longer. If you would rather hand that off, get a quote or read more about house cleaning services.