The other main reason behind the XR longeviety as compared to the newer 4 strokes is piston rings. As mentioned by cleonard, the newer bikes have very low profile pistons, but they also only have two rings, one compression ring and one oil ring. Most engines built for reliability typically have 3 rings, two compression, one oil. The reason for this is that as the oil viscosity begins to break down, and the rings fail to adequately retain compression, the fuel/air mixture will escape past the rings and be deposited into the engine oil. Over time, this accelerates the breakdown of the oil and eventaully will burn the rings completely. That's why the newer 4-strokes have to have the oil changed so frequently. The XR's came with three rings, larger pistons, and if they are porperly maintained and jetted as stated before, they won't overheat. My XR is a decade and a half old, has only had the oil/filter chaged regularly and has nary a problem, and according to my oil temp guage, the oil sits firmly between 150-200 degrees, even in the 100 degree heat here in Sacramento. If its longevity your looking for, an XR is your answer.Come on!! 150-200degs? My 650 is between 230 and 250degs and the 400 sits right at 200(stock oil cooler). I'll give that I have the 650 running a little lean but I swapped the two dipsticks (XR's Only) just for shits and grins and even with the different probe lengths they read the same.