i change my oil every 4000 km, i use shell rotella. the oil has about 2000 kilometers on it, and the cooler is clean. you pull my dipstick and the oil is clear not black. But today was a lot warmer outside and didnt have no problem at all. When it was -20 outside, driving down the road at 65 mph would make that cooler feel like -60, so im just assuming i got some real thick oil stuck in there cause of the cold weather.