Here are the cliffs notes on why...

1. Replaced A/C compressor, got car back...belt squealed. Tried belt dressing, didn't work..replaced belt problem solved.

2. Fast forward three months, this belt is squealing just a bit, add belt dressing...got worse...add lots of dressing, noise goes away for 10-20 seconds.

3. The wife and I bust our butt cleaning all the dressing off the pulleys, put on a new belt and problem solved.

Our mechanic, with 20+ years of experience said to NEVER EVER use belt conditioner/dressing on a ribbed belt. I guess the "safe for all belts" on the can label is misleading. The belt tensioner was examined by the mechanic and he said the timing marks are correct and tensioner is not "bouncing". The tensioner is $165.00, the belt was $36.00....shew, dodged a bullet.