A while back I came across some dedicated lock lubricant at a locksmith, but I can't remember the name at the moment. (isn't that always the way?)

You might try a local locksmith and see what they have, though.
WD-40 helps displace water from the door locks, but I don't think I'd rely on it over the long run, or in the ignition lock.

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...