Oil

As the engine wears over time, the piston rings and valve guides and seals become worn and oil begins leaking past them and into the combustion chambers. It then gets burned along with the air/fuel mixture and exits the tail pipe (fouling the plugs, 02 sensors and cat converters along the way).

With only 126K miles on your car, this would suggest your engine wasn't maintained very well (oil changes). If a new engine has it's oil changed every 3,000 miles, it should last 200K and sometimes into the 300K range. (I've seen engines with half a million miles on the original motor!)