I need to get my 90K miles service performed, and I started to think about changing the spark plug wires and the distributor cap. I recognize that Volvo doesn't specify changing the wires, and I can't remember a manufacturer that does. Yet mechanics almost routinely replace the wires, sometimes with pricey versions. What could actually fail with the wires? The insulation could break down which would cause parallel paths to ground, reducing the current to the plug, possibly preventing the spark from reaching it. However, if I was engineering the elastomer in the wire, I think I would make some bounding assumptions about the temperature under the hood over time, and use an ageing thumb rule based on known degredation as a function of temperature, and then I could specify when to change the wires. Yet this is not done, which leads me to the hypothesis that the wires really don't need to be changed except under extreme conditions or if they are damaged in some way. Anybody have any thoughts on this issue? Thanks.