Are you replacing the strut mounts or the spring seats? The strut mounts are the parts with the 3 studs that bolt onto the strut tower. There is a bearing in the middle. I don't think these parts fail very often...I have never replaced mine in 153K miles.

The spring seats are the large rubber bushing that sits on top of the coil spring and are notorius for going bad (the metal collar separates from the rubber). I have replaced mine on both sides over the years. The key is to buy an orginal equipment one, not the cheaper aftermarket one. The original equipment one I replaced has lasted 80K miles so far. I made the mistake once of getting the cheaper aftermarket one and it failed in less than 20K miles.

Good luck.
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2001 S80 2.9 (non-turbo), 150K miles