Reasons for Motor Oil Consumption
AMSOIL Inc. issued Technical Service Bulletin (TSB) MO-2004-04-03 several years ago outlining reasons for motor oil consumption. Forty reasons are listed, two of which are oil-related, and the balance of issues are mechanical in nature. (The two oil-related reasons are "dirty oil" and overfilling the crankcase).
It should be noted that a degree of consumption should be anticipated in all engines. What is considered normal or 
acceptable, however, will vary from one engine or application to the next. Some OEM's (original equipment manufacturers) will not entertain oil consumption complaints from customers unless consumption is greater than one quart in 800 miles! Other engines will not consume a quart of oil in 10,000 miles. Over-the-Road trucking operations are not concerned until consumption reaches one gallon of oil per 10,000 miles of operation.
All things being equal, engines using AMSOIL Synthetic Motor Oils will consume less oil because of AMSOIL's lower volatility and tighter seal within the engine. When one engine consumes less oil than another engine, and the quality of the oil is the same, the only variable is the engine itself.
Some engines do use "too much oil." And there are just about as many opinions as to why this occurs as there are vehicle owners. However, there are scientific, technical reasons for this phenomenon. This TSB provides insight as to the many causes of oil consumption in engines.
Read or download AMSOIL TSB MO-2004-04-03 "Reasons for Motor Oil Consumption"