Pretty straightforward question here...

When is the "correct" (or appropriate) time to R&R distributor cap, rotor, and wires? Do you wait until problems crop-up, or do you replace these items to prevent said problems?

As an aside, are there ways to prolong the life of these parts (i.e. cleaning or other normal maintenance procedures?)

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Eric

1991 560SEL-Euro (214,000 miles)
1989 420SEL (Retired from daily use at 325,000 miles; Use as donor vehicle)