i currently have an 03 deville. great car and reliable. very enjoyable to drive. Can red line it for hours. temp gauge has only gone 1 bar above the half way point once when screaming up one of the steepest inclines in the state. Currently seeking the right 2011 as I'd like a newer one.
yes every northstar leaks oil...and patching any leak they would have will cost you. any serious engine work is pricey as hell....thankfully I've had none. i recommend looking up head gasket repair, oil pan reseal, upper oil pan reseal, rear.main seal replacement, and.half case seal. if you're comfortable spending thousands on an engine that is definetly worthy ....then go for it.
change the coolant and t stat every other year one thing i dont like is that the tranny has a "lifetime: filter that can only be changed on complete teardown.
also....i hate to put this out there but racial profiling is a real thing with these cars. i own 4 different cars and between my wife and i, we have been pulled over 8 to 10 times in the last couple of years.....all in the cadillac. 0 times in other vehicles even if speeding or driving with expired tags......i get followed a lot too. when i roll down the window and put an arm out with a cigarette or something to show that I'm white....the following stops abruptly. I never believed it was a real thing until i bought this car.....reel cop magnet.....kinda died down since the whole defund the police movement started.....but we still get followed home bi weekly....mostly at night....always in town by local police.