[Lil bastard]

Your mechanic is correct. The Dipstick gives you a direct reading of the oil volume while the display merely interprets a signal from a sensor.

In TSB (Technical Service Bulletin) Boxster 2/99 1 1701, Porsche increased the Oil Filling Capacity spec from 8.7 Qts. (8.25 Ltr.) to 9.25 Qts (8.75 Ltr.) w/ Filter change.

Interestingly, there has never been a TSB issued on recalibrating the Oil Level Sender or DME. Nor has there been a revision of the Sender, or the Dipstick (PET shows the same PN for all model years). In other words, Porsche revised the oil capacity, but never modified the tools for measuring this revised capacity.

So, this revision automatically created a discrepancy between the digital gauge, the Dipstick and the fill spec. Presumably then, none of them are truly correct. A correct fill should read (theoretically) slightly higher on both the Dipstick and the digital gauge. In the case of the digital gauge, where each Bar represents roughly 8oz. or 1 Cup, you should now fill to the Top Bar (formerly the Bar beneath), but it also means there is no longer an indication of an overfill. Overfill a little, or a lot, and the gauge will read the same.

But, realize that there is an 'OK' range of 7 Bars (or approx. 1.75 Qts.), and OK means OK. This range gives you a lot of lattitude, while still being able to safely operate the engine (personally, I would fill to the Bar below the Top to insure there is no overfill).

It's usually better to run a little 'low' (less than Full), than risk overfilling the crankcase by obsessing about keeping it 'Full'. Overfilling can cause the oil to foam and risk oil starvation to the bearings as well as issues with the AOS system. Keep it in the OK range and you'll never have any issues.