Porsche Boxster Owner's Manual page 216

3. Unscrew cap of the oil filler opening.
4. Add at most half a liter of engine oil at a time.
5. Measure oil level again with the on-board
computer.
6. Add more engine oil if necessary. Never add
more engine oil than required to reach the
max. mark.
7. Close cap carefully.
8. Close service flap.
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Maintenance, Car Care