How come when you change motor oil, you need to drain it first?

If the motor oil stays in the oil tank (not sure if that's the right term), doesn't it mean that it's unused? How come when you change motor oil, you need to drain it first?
Without getting into technical details, the oil gets dirty and ';worn out';


when it lubricates the engine's moving parts. That's why it has to be changed out.





The rule of thumb for ordinary (non-synthetic) motor oil is to change every 3000 miles. The past 20 years, many manufacturer's have been saying you only need to change the oil every 4-5K miles, but they're are generally in the business of selling cars. Changing your oil more often will not hurt your car. Its also important to change the filter when you change the oil, as the filter is full of the used oil when you change it.





The used oil can be cleaned and recycled for other uses.


How come when you change motor oil, you need to drain it first?
Ive got to laugh when I get asked this one, ok where the oil stays is called the oil resoviour. The oil sits in there until you start your engine, collects water, also called condensation. As your engine runs it picks up all types of abasives that come off the inside of your engine, it gets back to your so called oil tank, some get filtered by the filter, some not. The so called VARNISH that gets through has to be removed from the oil. The oil filter will not remove this. This is why the oil also has to be drained, City driving, every 2000 miles, highway driving, every 3000, and dont forget to put water remover in your gas tank to reomove condensation in the gas tank once in the spring and once every tank in the winter, this way your car will start all winter long.............bye
Ruffles, In most cars the motor oil occupies space in the engine oil pan on the very bottom of the motor. The oil pump has a tubular extension on the bottom that extends down near the bottom of the oil pan called the ';oil pick up screen.'; When changing motor oil, there is a drain plug on the side of the oil pan near the bottom. The drain plug is threaded into the oil pan and removed to fully drain all the old used oil out. The oil filter is removed from the engine block at this time also. It has a large threaded hole in the center with smaller holes for pressurized oil circulation inside the oil filter sealing gasket.





The term changing oil in your mind should be ';exchanging the motor oil';. That's what we do, drain out the old beat up oil filled with contaminentcontaminantsewing the oil drain plug back into place, and installing a fresh oil filter. At this time the oil storage pan is empty. We add however many quarts of oil required back inside the engine through the oil fill hole usually on a valve cover. It is free to drain back to the oil pump pick up screen where it's good to go till the next oil change.





Correct oil changes are determined by mileage and or time.
The oil continuously circulates through the engine and oil tank (crankcase actually). All of it gets used and has to be changed. The oil pump draws oil from the crankcase, sends it through the filter, then it goes throughout the engine before it's returned to the crankcase for another cycle. Your car holds between 4-6 qts depending upon your engine.
your oil stays in an oil ';pan'; at the bottom of the engine. a pickup tube sucks it in from the pan and circulates it to the various engine components. over time, blow-by and contaminates deteriorate the quality of the oil, and the old stuff needs to be removed. this is what they do on the lift at the oil change place. after it is removed, they plug the pan with a bolt and replace the oil filter. they will install new oil through the fill hole at the top of the engine. this drains down into the pan, ready to be sucked up again.





if you just add it and add it and so on, it gets too full and the crankshaft will splash the oil and cause air bubbles to form, which also gets sucked up, it's called ';aeriation'; and it's bad for the engine bearings. they don't like air. it starved them of crucial lubrication.
Your oil pan (right term) on the bottom of the engine keeps most of the oil. It returns to the pan after lubricating your engine and is usually filtered to be used again. This includes dirt and grime it picks up. So you must drain the dirty oil before pouring in the new.
The oil is constantly circulating in the engine, but the oil stays THE SAME LEVEL in the oil pan. When you change the oil, you need to drain all of the used oil, change the filter, replace the oil plug, and refill with new oil.