My experience with the black plastic ones has been the "whitening" problem that occurs when certain solvents are used to clean them. You often find these der.s on bike-boom bikes that have a ton of greasy sludge on them, so the first thought is to dunk them in a your parts washer. But experience has taught me to use something milder. (I use simple greens spray bicycle cleaner, I think) on them now.. I have tries "de-whitening past "mistakes" with Armour-All, with limited success.
Mark Hoffman
New Britain, CT
USA