The most common causes of excessive engine oil consumption in cars or trucks or other vehicles is either due to worn out valve guides and/or worn out piston rings.
As the engine racks up miles on it the valve guides and piston rings eventually get worn out which causes engine oil to get past the piston rings and then into the cylinders.
Then the engine actually burns off that excessive oil as a result of it getting ignited with the gasoline.
Oil also gets past the worn valve guides which then leads to it getting burned through the exhaust.
If your engine is smoking a lot from the exhaust and you're burning oil then the engine is likely in need of an engine rebuild.
Sometimes it can be worth the cost to rebuild an engine and other times it may not be worth it depending on the value of the vehicle.