An engine can lose oil in other ways besides leaking externally. The most common ways oil can be lost internally is either through a defective PCV valve, or through worn piston rings, or worn valve stem oil seals, or worn valve guides.
Oil loss can also be caused by a leak in the cylinder head gasket; which opens up a path between an oil passage and a cylinder; or from a head gasket leak between an oil passage and a coolant passage. These two issues are usually caused by the engine having overheated in the past; but sometimes they can appear in an engine which has never overheated; as a result of an engine design weakness.
Other causes of engine oil loss could be either improper break in when the engine was new, or the use of synthetic oil before the engine became fully broken in, or the use of unsuitable oil types or mixing of incompatible oil formulas. Some fuel or oil additives could also ruin an engine's oil sealing properties, which would lead to oil consumption past the piston rings.
It would probably require the services of an experienced, competent, honest mechanic to sort out these possibilities. However; if you have owned this car since it was new; and it has always used oil at this rate; it probably would be due to an improper break in or lubricaton issue. This type of problem would typically require an overhaul or engine replacement to correct.
If the car did not use oil when it was younger; but suddenly began using oil at one point in time; it most likely was either caused by an error when it was serviced (such as switching to an incompatible oil brand or type; or the use of an additive which harmed the engine); or a head gasket design flaw which manifested at that time.
If your car now has more than about 60,000 miles on it; the valve stem oil seals may have deteriorated and need replacement. This issue usually causes a cloud of blue smoke to come out of the tailpipe when the motor is first started. This repair in not very expensive; and can make a significant difference in oil usage.
If your car has a manual transmission; excessive oil consumption can be caused by unskilled driving; in which the motor is "lugged" by driving it too slowly in the gears. A general rule is to not use second gear below 15 mph; not use third gear below 25 mph, not use fourth gear below 35 mph, and not use fifth gear below 45 mph. When climbing steep hills; a lower gear must be used than when driving on level ground.
If you have been adding different brands or types of oil when the level became low; this can greatly increase the rate of oil usage. The most important thing you can do to minimize oil usage in any engine is to select an appropriate brand and grade of oil; and make sure to only use that particular brand and grade of oil whenever any oil is put in the engine; regardless of whether it is just topped up or completely changed. Oil companies promote a dangerous myth that oil types can be mixed indiscriminately. This is absolutely false!!! It is also necessary to regularly change the oil and oil filter within the car manufacturer's recommended time and mileage intervals.
If you are uncertain about the oil brands or grades to use; or would like a recommendation for a local mechanic; please click the "answer this question" button below this response, and post a message in the box that appears. Indicate the brand(s) and grades of oil you've been using, and any additives that have been used, as well as the length of time you've owned this car, and the total number of miles it has gone. If you'd like a recommendation for a local mechanic; please include the city and state in which you live. Then click the "submit answer" button. Thank you.