On 11 October 2011, Herman Jorens in Belgium wrote:
"On our cars we use different washers in copper or fiber. So when do you use fiber washers and when copper washers"?

Fiber washers can be used for near zero pressure fluid seals, like on the downstream side of the oil pressure relief valve, tappet cover bolts, gearbox side cover bolt (one), steering rack dampers, carburetor overflow banjo fittings. Copper washers are needed to seal higher pressure fluid joints, like brake line banjo fittings, spark plugs, and various water and oil plugs in the engine block.