I'd like to add that plasticizers are used to soften the polyurethane. fuel/oil will eventually break down and harden these plasticizers. it has the same effect on rubber. Which is what happened to my UPP intakes (they were older). We have similar problems with lane oil getting onto our distributor belts. bushings, lift wheels and other components both polyurethane and rubber based in the bowling industry. they eventually will harden, curl and crack. which is why we have to clean machines often with a cleaner that is not aggressive (I use windex) If there is constant stressing/moving with hardened plasticizers it will crack.
Not saying one is better then other.. They both have the same demons. I just like the stability of the billet pieces.