Yes, there is 'oil' in coffee beans. When the beans are roasted, the oils are released. It where, from what I've been told, a lot of the flavor contained.

It's called "red-eye gravy". It's especially popular in the south. There are a ton of recipes on-line.

If it has no milk in it, just warm it in the microwave. Why not? But, I do love a cup of fresh brewed the best.

I accidently drank day old coffee with milk in it?

I accidently drank day old coffee with milk in it?

Coffee beans naturally contain volatile oils - the roasting process causes the sugars in the beans to carmelize, and that's a big part of the aroma. When you add hot water to ground coffee, the remaining oils are separated from the beans and are part of the finished brew.
It's not all that different from making pan gravy - the little bit of oil left in the pan mixes with the water, but if you let it sit, it forms a skin because the oil separates from the water. The only difference is that you don't put flour in your coffee.
If coffee is allowed to sit, the oils separate from the water, the suspended solids precipitate out, and there is no way to make them go back together again correctly. You can re-heat, and you can stir, but it still doesn't taste the same.

You won't notice "pond scum" on day-old coffee, but if you look at it from an angle, you'll see the same rainbow effect you see when you put a few drops of motor oil into a puddle.

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I'm most likely to drink old coffee if I make a glass of iced coffee - Texas, so it works nearly year round. And sometimes in the afternoon, I'll pour a small cup of water into the coffee pot (using the ground's from morning) and the coffee that's brewed at that time, added to what little is left in the pot from morning, usually makes a decent cup. I do add sweetener and milk, so I might feel differently if I drank black coffee.

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I like iced coffee so once I am done drinking it for the day I put the leftovers in the fridge and drink it cold the next day. I don't like it microwaved.

Ew no, no leftover coffee for me! I usually use the french press but if I do make a pot in the coffee brewer, it goes immediately into the carafe. Still needs to be drank within a few hours (I'd say about 3 probably).

Whichever pot I use, french press or the drip pot, goes into the dishwasher each evening. I also put into the dishwasher the press if I use the press pot, or the grounds basket and filter if I use the drip pot.

And the drip pot gets the vinegar treatment monthly. I'd say on average we use that pot twice a week.

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I drink day old coffee all the time, I hate to waste it. If I don't drink it I freeze it in ice cube trays, that way when I want a quick cup of coffee I add a coffee cube and it's not watered down.