Short anwser: Never put butter, oil, etc, on a burn. This would worsen the burn.
Putting butter, oil, or anything else would trap heat and make the burn deeper. It would also make further treatment harder and would make the risk of infection higher:
Don’t apply burn ointments. Like butter (or mayonnaise), these ointments, usually oil-based, won’t relieve pain but instead will trap heat, slow down healing, and increase the risk of infection.
What to do instead: Once it happens, a burn is actually "cooking inside", and this for a while after the exposure, so the first thing to do is to stop this internal reaction using flowing water.
According to official guidelines for general public in France (page 12-14), and sticking to thermal burns here (not chemical nor electrical), you should:
Quickly water the burn
- Act right after the burn if safely possible. (You are not supposed to put yourself in danger to save someone: this is the best way to end up with two victims instead of one)
- Put the burn under flowing water, between 15 and 25° Celsius. The tap is fine to to that, just keep in mind to keep the water pressure low enough not to press the burn.
- While watering, remove the clothes of the victim unless they adhere to the burn (should they adhere, don't pull on clothes).
Evaluate the burn
- No blister or blister smaller that 1/2 of the victim's palm ; far from natural orifices ; not on the neck, face or joints ; just red, not black and white: this is a minor burn
- Any burn that does not match the criteria above, or any large red area on a children, is a major burn
Should it be a minor burn
- Keep on watering as long as the burn hurts and the victim doesn't feel cold. Not just a couple of seconds: 5 or 10 minutes is not surprising, it's actually fine. Watering is what prevents (more) blisters from appearing later on.
- Depending on the pain, aspect and victim's age: watch over the burn or ask a doctor or a physician.
- Don't pierce blisters should there be any. Cover them with a sterile plaster.
- Should blisters, fever, heat, pain, etc, appear later on, ask for medical advice.
Should it be a major burn
- Call the medical emergency services, and, without hurrying (very important! Being calm is gaining time.)
- Tell your name, phone number, current location,
- Tell this is for a burn, and tell the victim's gender and age
- Describe the result of the evaluation you made.
- They will guide you in accordance to the protocols applicable in your country. This guidance will certainly include watering, covering the victim if he/she feels cold...)
- Do NOT hang up the phone before they tell you to do so
- Keep on watering as instructed by the medical assistance.
- Make the victim to lie down in a way he/she cannot fall (floor, bed...). Sitting down is OK the victim has difficulties to breathe. The victim should NOT sit on a chair but on the ground to prevent him/her from falling.
- Protect the victim with a clean linen. The burnt body parts should remain visible.
- Watch over the victim until the arrival of the ambulance.