Looks like the Victor Reinz Valve seals are made with Viton®:

http://www.engineparts.com/newsstories/020522_VR_953_Kits.pdf

Another link for whomever it may interest:
http://www.babcox.com/editorial/ar/ar79842.htm

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1989 300CE Astralsilber/Schwarz
1992 300CE Schwarz/Schwarz