I would use a wheel brush (resembles a bottle brush with a plastic instead of metal handle)with ordinary car wash soap. Stay away from wheel cleaners as many of them contain caustic chemicals that could damage the finish. Use plenty of water for rinsing and finish up with a coat of wax.
The earlier W123's had the chrome and painted wheel covers. You did not state whether yours was the Alloy wheels or the steel wheels. Me thinks the alloys on your car would have ben an upgrade by a previous owner, but I'm not positive of that.
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