Back in '99, Porsche revised the engine oil capacity for the M96 engine from:
Oil Change w/o filter
- from: 7.75L (8.18Q)
to: 8.5L (8.98Q)
Oil Change w/ filter
- from: 8.25L (8.71Q)
to: 8.75L (9.24Q)
This change was noted in TSB# Boxster 2/99 1 1701
Prior to this change, the proper fill level was 1 Bar from the top on the digital gauge and to the 'Full' mark on the dipstick (where applicable).
The problem is, while Porsche changed the capacity, they did NOT change the means by which to measure the proper level. There has been no revision to the digital Sending unit nor have they released a dipstick with a modified mark to reflect the change in capacity. Pretty stupid if you ask me, but there it is.
So, according to this TSB, running at the revised capacity, you should be filling to slightly over the 'Full' mark on the dipstick, and to the top mark on the digi gauge (though this is impossible to really measure because there is no longer an 'Overfull' indicator). In other words, there is no longer an acurate way to measure the 'Full' level at all.
Realize that the digi gauge gives you a very generous OK range of 7 (formerly 6) Bars. Each Bar represents 1 Cup. This means that the OK range is equal to 1.75 Qts.
Obsessing about getting the crankcase 'Full' risks having an overfill which can damage the engine due to foaming of the Oil, etc.
If you stick to the old marks on the guage and the dipstick, while you may not be running at the absolute revised capacity, you will nonetheless remain well within the OK range.
If truly anal about it, you can always measure exactly how much oil you take out when draining it (including the filter) and refill only that amount. Just be sure to give the car at least 20 min. drain time to be sure you get out most of the old oil.