How often should I change the oil on 1998 Boxster ?

[madmods]

If you drive it lower than 3,000RPM then you are 'okay' to change at 10k interval.

If you drive it in the 4000RPM range then you should change at 5k interval.

If you like to push it at red line occasionally, then you should think about changing your oil every 2.5k interval. Especially during hot summer

Hope this helps