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IMHO, dirty or moisture contaminated oil is the most common root cause of failed IMS bearings. My theory goes like this.
The original IMS seal degrades the longer it is in contact with degraded oil. When the seal degrades, it develops small leaks and the oil mixes with the bearing grease. The combination looses its ability to lubricate the bearing. The ensuing friction causes the bearing to fail. If I'm close in my theory, it would explain is why cars with high mileage intervals between oil changes or lightly driven cars that accumulate moisture see more bearing failures.
In light or my theory, one can take two actions to lower the rate of IMS bearing failure. Change the oil frequently, i.e., every 3 to 5 thousand miles is one. The other is to make sure the car is driven frequently enough at operating temperature to less the amount of moisture in the oil.
If I were buying an older car with low miles, I would upgrade the IMS bearing just because I wouldn't know whether the bearing seal was in good shape or not. My 01S DD has had it oil changed every 5k and is driven every day. At 105K miles, I think the seals are probably in good enough shape that I'll do the IMS upgrade when I change the clutch -- probably at 125K.
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