I know this reply is a bit late, but I figured I would add my two cents. The way the S40 was designed is that the Pads and the Rotors are of a softer compound so that the car stops quicker (that explains the unbelievable stopping power). Because of this, your rotor's and brake pads get worn out quicker. It is suggested that everytime you need a brake job that you replace both the rotors and the pads for the sheer fact that if you replace just the pads, the second set will wear faster because of the way the rotors have been used and warped. Volvo has updated their OEM parts now so that the Rotors and Brake pads are a bit longer lasting, but if you want longer life out of your brake pads and rotors definitely go aftermarket. For brake pads I strongly recommend Mintex. They have done very well for me. As for rotors I use the power slot rotors. I have had very good luck with them. Zimmerman also makes a good rotor.
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2000 S40 -- 80,000+ miles