When a car is topped up with oil, it clear to most people that you are supposed to fill it to the recommended amount as specified by the manufacturer (normally on a dip stick).
What exactly are all of the disadvantages to running an engine when the oil becomes too low?
Engine oil does much more for an engine than lubricate. It provides cooling, cleaning, and a bunch of other chores. You already know engine function is degraded when you run out of oil. Let's see if we can run it down to make more sense for you how it happens.
Let's say, for lack of argument, your engine is running with very little to no oil. The engine oil light is on as the engine is running. Antifreeze is up to snuff, but that won't stop overheating for too long. Here is what happens:
As whatever limited oil is in the engine is pushed around, it becomes hotter and hotter. It becomes hotter because the oil isn't allowed enough time to cool down a little before it is required to go back into the engine and do its thing. As this happens, the oil will wear out faster. Fully synthetic oil will last longer than dino (refined) oil, but it will suffer defeat sooner or later as well. Several reactions happen to the oil, to include oxidation, thermal degradation, and compressive heating. Mind you, all of these things are created under normal use conditions (and thus the reason we change oils when we should), but under extreme conditions as you've suggested, it all happens much faster.
This is a great image which describes what happens from the Machinery Lubrication website -
All of this breakdown creates the regular: tar; sludge; varnish; soot; etc. All of these things start depositing on the internals of your engine, such as the rings, bearings, cylinder walls ... anywhere oil would normally flow. If you go back to your chemistry teachings, you'll probably remember that a large portion of oil is carbon. All of the previously mentioned nasty substances are actually a carbon byproducts. As these carbon substances continue to deposit, any part which comes in contact with them gets wear on them as carbon can become quite hard and/or sticky under the right circumstances. This starts creating more friction on these parts. So now instead of cooling/lubricating/cleaning, the oil has broken down is now heating (due to more friction), creating deposits, and making the engine a really filthy place. You should realize it is now doing everything it was originally meant to replace.
Every machined surface within the engine is susceptible to the increased wear. As the oil continues to break down at an ever increasing rate, the temperature inside the engine on these machines surfaces continue to get hotter and hotter. Wear occurs faster and faster until something gives. Most of the times in these situations, the first thing to go is a rod or main bearing. If the backside of the rod bearing is strong enough when this happens, you can throw a rod.
Throwing a rod is basically where the friction on the bearing at the rod journal of the crankshaft is strong enough to overcome the strength of the rod itself and it has a catastrophic plastic deformation of the metal. The piston can go down the bore and will not return. It can go out the bottom and then gets flung around inside of the oil pan, or the rod itself will break and force itself and/or the piston out the side of the engine block.
At this point, there is no coming back from it (in most cases). The engine is done and will need to be replaced.
If you continue to run the engine with very low oil, destruction will occur ... and no ... I'm not being melodramatic about it.
EDIT: Here are a couple of videos which describe what happens when you don't change the oil in your vehicle. This is what happens with oil when there isn't enough of it ... with lack of oil, it doesn't take 130k miles to look like this.
In most engines, when you fill the oil, it drains into a reservoir on the bottom of the engine known as the oil pan. The tube for the dipstick goes right down into this reservoir to measure the level of the oil. The other important item located in this reservoir is the pick-up tube for the oil pump.
As long as the pick-up tube for the pump stays under the oil level, then the answer to your question is "not much". You might loose a little cooling ability because of the reduced volume of oil, but everything will work correctly, and you won't see any warnings because your oil pressure will be fine.
Also in most cars, the oil warning light is triggered by a drop in oil pressure, which can be caused by your oil level being low. For this to happen you're oil has to be VERY low. I've been 2 quarts/liters low before and only saw my oil light come on when taking a corner at a fast speed. The little oil left in the oil pan would slosh to the side, and the pickup tube would go dry, and pressure was lost.
You should NEVER drive a car if the oil light is on or if oil pressure is not normal, but running low on oil isn't going to hurt until it gets too low. Once you loose oil pressure, damage can be close behind.
Engine oil is used for lubrication and cooling, and a drop in pressure means that oil isn't going to get everywhere it needs to be, or at the very least, it's not going to cycle through fast enough and it will start to overheat and break down which will also cause a loss of lubrication.
Disclaimer: I work with an older 202 straight 6 Holden engine in a landrover, so my perspective is somewhat vintage. However the basic underlying concepts are the same.
Cause: Low oil in the oil pan, at the very bottom of the engine. Normally the oil should read right on the Full line on a dipstick, when the vehicle is level and the motor is cold and has not run for several hours. This allows the oil in the upper part of the engine to run back down into the pan.
Explanation: The oil is drawn from the pan into a pipe called the oil pickup. This is a hard pipe that hangs down into the lower part of your oil pan. As long as this part is covered by oil all the time, you're good. On a steep ascent or descent, the oil may move away from the pickup, which is the same condition as when there's low oil all the time. Also, hard cornering can slosh around the oil so the pickup comes clear for an instant.
Here's an oil pickup and screen in place. The pan is not there, and this motor is upside-down.
If the pickup is uncovered, it will attempt to suck in crankcase air rather than straight oil. Since air is far more compressible than oil, the effective oil pressure drops with air in the line. The oil pump should push through most air bubbles, but in the worst case it may end up empty of oil, and filled with air instead. This could cause an air lock and oil simply stops flowing.
Effect: Your motor pumps the oil up the side of the engine into the head, where there are lots of moving parts like cams and cam followers. My motor has push rods and rockers and valve stems that all need to be lubricated continuously. With a lower oil pressure from the pump, less oil gets pushed to the top of the engine and these moving parts can run drier.
In addition, there are oil throwers and slingers at various places around the main crankshaft. These catch and scoop oil and throw it into/onto places that need lube, like the cylinder walls when the piston is up, and into the connecting rod eye bearings. Should the oil level drop low, this lower-engine lubrication is reduced, and things run hot, with increased friction and wear.
I recall that the average car engine cycles its entire store of oil through the engine 4 times a minute. I suspect its more often in a more recent engine.
Source - this is worth watching: http://www.youtube.com/watch?v=CDXXkpGiLU0
Its not all bad - there are more modern systems like dry sumps in racing cars, where the oil is pumped into a holding tank under pressure and then fed to the engine, so its got a reservoir of oil for the corners. You still shouldn't run out, but you can hard-corner without loosing lubrication.
Story: My other-half's car was sounding very tappity, but otherwise ran okay. Wasn't till she was driving the car up a decent grade that the oil pressure light came on. It was 2 litres low, and barely registering on the dipstick at all. A couple days later, she drove the car out of the garage and then checked the oil. It read real low because the motor had just been running. So she put an extra 2L in and drove for 30 minutes. Car was running really badly, blowing blue smoke as the oil was forced around the rings past the pistons and into the combustion chamber. We drained some out and all was well.
So its a case of too much is bad, not enough is bad, and just right is just right.
I can answer this because it has happened to me a few times. There are several different possibilities depending on the ambient temperature and how old the engine is. Here are three real scenarios that have happened with my 1996 Volvo 850:
(1) When the oil gets low the first thing you might notice is more noise. Oil muffles the engine, so when oil gets low the clatter from the engine becomes slightly louder. Eventually the "low pressure oil" oil light comes (hopefully). If the car is new, it will get hot rapidly because the fitting between the cylinders and bore is tight. If the engine is old, it may just keep puttering away because of a looser fit. As long as the temperature does not get hot, it is ok, but your engine will have accelerated wear.
(2) In high ambient temperature (summertime), you will get a temperature overload first. Once my low-oil-pressure light failed so I ran low on oil without noticing. The temperature of the engine heated up and I was not watching the temperature indicator. The first thing I noticed was steam coming out of hood. I IMMEDIATELY pulled over. With a full-sized beach towel I opened the hood, and removed the radiator cap, steam and hot water exploded out, gushing over the engine; this is good because pressure release causes cooling. I then got a gallon of cold water and poured it in and poured two more gallons right over the engine. No permanent damage was done.
Edit: note that this can be extremely dangerous! Coolant is under pressure and opening the cap on a hot engine can cause it to start boiling and expel hot coolant with considerable force. Serious burns can result. The standard recommendation is to allow the engine to cool before removing the radiator or coolant reservoir cap. In my case I used a large, full-sized beach towel to stifle the explosion. Using such a large towel, blanket or other large, thick fabric covering is necessary to avoid burns.
(3) In a low temperature situation (winter) in an old car, the engine may seize before overheating. This happened to me a few weeks ago. The darn oil pressure sensor has failed again. I was happily driving on the highway, no temperature abnormality at all, but ignoring engine noise which should have tipped me off I was low on oil, VERY low. I pulled over to refuel. I filled gas tank up and went to start the car: putt, putt, putt, it went like 5 feet and then stopped. Tried again, prrrr, prrr, erk, stall. The engine was seized. This means it is hot and the parts are rubbing. On the highway, the pistons are moving so fast and hard, they keep going, but once stopped, the friction prevents them from moving: seizure. I checked the oil: gone completely. I filled up the oil with the right amount. The car started and drove normally.
Because my car is older it can tolerate higher temperatures somewhat and not overheat as easily. With a newer car with tight-fitting pistons or a performance car, loss of oil is much more dangerous because the engine can build up heat so fast it becomes red hot. If this happens, the head can warp and then the engine will leak oil rapidly. The engine is more or less garbage at that point.
The entire lubrication system for the engine fails, leading to scratching of the cylinders and the barrels, after which point they dont run smoothly and degrade much faster. the pistons are made of aluminium and the cylinders have a special tough glassy coating so they screatch very easily and when the engine is too hot the aluminium deforms and heats in a vicious circle until they lock up or it overheats and smokes.
The engine will likely seize up.
This happened to me with a lawnmower once. I tried to mow the lawn after dad drained the oil, but before he filled it. It ran for a few minutes, then stopped as if the blade had hit something hard, but without the sound of the blade hitting something.
We got very lucky, and were able to start it up after adding oil and letting it cool.
It's not good. The little oil you have left will overheat & degrade as others have explained better. With your oil level really low like that, as long as the engine isn't overheating yet, you might be able to get the car to lug itself 5 or 10 miles to a shop to get more oil w/out problems, but it's already really pretty harsh on the car
Basically, your mileage will get worse (maybe permanently, depending on the tars & resins formed by the heat-degraded oil), & your engine will suffer exponentially more wear than usual, b/c the oil can't do its job well enough any more
If you kept on driving far enough beyond this point your engine would eventually overheat & be destroyed - either coming apart from plastic deformation or even welding itself shut into a very expensive piece of modern art. Under extreme overheating, thermal expansion becomes significant, so you get a positive feedback loop w/ more & more heat & friction. If the internals hold up, this will cause it to seize, and if enough power & momentum are involved it can weld the pistons to the block, kind of like how some wheels are made by friction welding
Gap between cylinder and engine core is really really small. This gap is filled with oil under normal circumstances, allowing cylinder to "slide" in its container.
When you run out of oil, friction between cylinder and its container increases a lot, causing high wear.
When car runs without oil, friction between cylinder and its container generates a lot of heat, causing further cylinder wear. At the critical point, cylinder gets so hot so it actually 'melts' into its container and locks up with unability to move. Unfortunately, all cylinders are hard-joined to crankshaft and engine momentum is too high to stall immediately (as other cylinders are still making explosions and generating power), crankshaft usually disjoints from cylinder and damages some random part of engine by breaking hole in it, turning engine into junk.
When this happens, it usually looks like some sort of "explosion" where engine makes loud sound (breaking crankshaft) and holes emit large amount of smoke from burned fuel, oil and hot metal of cylinders.
Please note that while this depends on car, it is really small time frame engine can work without an oil, it may be around betweem 30 seconds and few minutes at max. So when you see oil pressure warning in cockpit, stop car immediately and turn off engine.
Without oil to lubricate, metal on metal surfaces begin to heat up from friction. The extra heat causes the metal to expand. When it expands enough, some part of the engine will try to stop moving, but the other parts may not stop yet because of momentum or power, causing bends and breaks of internal engine parts.
When oil is too low, the oil pump might not always be able to draw oil in, leading to gaps in lubrication where the extra heat will burn and break up whatever oil did remain, accelerating towards eventual failure.
