Porsche 928 ST1991 Owner's Manual page 152

The recommended service intervals apply under normal driving conditions. If you drive mainly in dusty areas, check the air cleaner element
more often and replace if necessary. The conditions of oil, and wear-and-tear items (such as tires, brakes, clutch lining) depend greatly on the
amount of driving and on driving habits. Therefore, oil and wear-and-tear items should be checked more frequently, and if necessary replaced
at shorter intervals. Also, the battery electrolyte level should be checked more often. A complete maintenance and lubrication service should
be performed at least once a year, preferably before the winter. The same applies to protective undercoating for the vehicle.
Exchange and new parts require the same servicing as components of new vehicles.
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