Puny Chipmunk Square Koala Mechanic (mostly shade tree but also pro for a couple of years) and son of a pre-WWII SoCal hot rodder / aircraft / missile machinist / mechanic here: I've always changed oil every 4k miles (~6,500km). With modern synthetics, it's probably overkill, but the engines I've rebuilt that I've treated this way have never had a drop of sludge in them. I always add 0.25qu (~0.23l) of the old Ford FA ATF to the oil. It is high detergent and helps keep things clean inside the engine.