I see there is a thread on O-rings running in two places, so I trans-post what I said on the other thread here:
Until l researched this a bit, I thought a green CR or neoprene Orings used for refrigerant applications would work as well in place of black rubber rings (see below). Viton appears the best material.
From
http://www.sealdynamics-oem.com/gas...c-materials.htm
Fluorocarbon, Viton, Fluorel, or FKM
Fluorocarbon combines resistance to a broader range of chemicals than any of the other elastomers. It constitutes the closest available approach to the universal O-ring elastomer. Although most fluorocarbon compounds become quite hard at temperatures below -4°F (-20°C), they do not easily fracture, so they are serviceable at much lower temperatures. Fluorocarbon compounds provide a continuous 437"F (225°C) high temperature capability.
Advantages:
Excellent chemical resistance
Excellent heat resistance
Good mechanical properties
Good compression set resistance
Disadvantage: Fair low temperature resistance
Chloroprene, Neoprene, or CR
Neoprene is a polymer of chlorobutadiene and is unusual In that it is moderately resistant to both petroleum oils and weather (ozone, sunlight, oxygen). This qualifies neoprene for O-ring service where many other elastomers would not be satisfactory. It is also used extensively for sealing refrigeration fluids. Neoprene has good compression set characteristics and a temperature range of -67"F to 284°F (-55°C to 140°C).
Advantages:
Moderate weather resistance
Moderate 0 resistance
Versatile
Disadvantage: Moderate solvent and water resistance
FWIW. Another reference
http://www.oringsusa.com/html/fluid_compat.html says Neoprene is marginal when used with 10W40 (engine lubricating oil). I would therefore avoid using a green O-ring on a MB Filter stem.
Green is used primarily for identifying a material compatible with refrigerants. High tept material is Silicone, a reddish brown color but may also be available in other colors.
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1971 220 (gas) 4-spd manual 106441
1979 300TD w/ ’85 turbo engine 296650
1983 300D 243280
1985 300TD 223470
1987 300D 262300