X2 - what the rubber is made of (how much sulfur etc.), what vulcanizing temperature, and how much it's compressed to fit the space all play a part.
For the same rubber volume, a band 1/4" wide and 1" in diameter will be much softer (less resistance to movement) than a band 1" wide and 1/4" diameter.
I've heard (and believe) that a too-soft rubber insert can be stiffened somewhat by driving wire brads through front to back in a regular pattern (like 6, 1 every 60° etc.) - it works on suspension bushings.
The reverse (too stiff) can be done by drilled holes in the same axis. This also accelerates rubber deterioration.
I don't think either method will help an old damper with the rubber decayed by ozone, oil vapors, heat cycles etc.
Again: the dangerous stuff is where:
1. it's coming apart - eventually, it will come off and rip out your hoses, then go through your radiator
2. the pulley mis-aligns and ruins your fan and pump drive
3. you run it too long at the same RPM which causes "finger tingle"
