If you have old oil in your vehicle, then it is likely getting dirtier and thicker. This will make it harder for the oil to lubricate your engine's components effectively. As a result, your engine will begin making various metal on metal noises because they are not getting lubricated properly.
Any engine oil that has been used inside of a vehicle is considered used. This is because during its use the oil comes in contact with metal particles, dirt, water and other impurities which can contaminate it.
In most cases, taking your oil to a recycling center or business will be at no cost to you. In some cases however, like California, they will even pay you when you drop off your oil. It is worth mentioning this is usually less than a dollar per gallon, but still a nice incentive.
The best way to store used oil is to put it in an air-tight container that can be sealed with a screw on cap. Once you’ve done that, you can take it to your nearest recycling facility, auto-store that offers recycling, or in some cases, even schedule a pick-up.
If you’re looking to get creative with your used oil, remember that it still has lots of life left and can be used for other purposes. There are tons of great ideas online showing off unique uses for motor oil; use in an oil burning furnace (many people on craigslist are always taking oil for exactly this).