Does the engine appear to run OK? If yes then I would do at least ( 2 ) 500 mile oil changes. This will loosen up a lot of the sludge in the engine. As long as the oil passages are not clogged, the oil will clean the engine.
I've read of people using transmission fluid in place of oil for a few hundred miles as the ATF has some powerful detergents. IMHO, if the ATF does its job too well, it might free up a large chunk of sludge and clog up some important oil passages.
Why do people treat their vehicles so poorly? If you adhere to the maintenance schedule, that Ecotec engine will outlast the body of the car.