As KM mentioned. I have soaked mine in oil. I even used the dirty oil from an oil change though.
Let it sit in the oil for about an hour, then hang the part over the pan to let the oil drip off overnight. Wipe clean and it looks like new.
Theory behind it is that plastic is a petroleum product so giving it the oil shot is like 'replenishing' it.
Patriot Guard Rider
2000 VN750 Sere (Serendipity)
1990 GL1500 (Ole Blue)
1986 VN750 EVie (project bike, heavy custom)
VROC # 11628 / 25000-H
ARVROC # 12 Coordinator and Crowd Control
OKVROC # 18 (H)
TNVROC # 45 (H)