replaced aos still smoking, please help :-)

[Jamesp]

A couple of thoughts:

You didn't specify mileage - if 300K the engine is worn out.

Additionally, if the oil was neglected over a long period or if it sat for an extended period the rings may be sticking, this is common in other smoking cars so I'll assume it is in Boxsters. The trick I learned (and have successfully used) is 3 oil changes at 500 mile intervals. Skip cleaning additives as they destroy the lubrication properties of the oil, the quick oil changes work by refreshing the detergent in the oil which cleans the rings.