According to some experts (Raby / Hartech in the UK), bore wear is the biggest culprit for oil usage, though to be honest, its the bigger capacity engine cars that suffer most. By what I've read, either the bores go oval over time or scoring occurs and high oil usage is the result.
Some of the later 3.6 & 3.8 litre engines suffer scoring of the Locasil coating & pistons which looks like seizure marks along the bores and is accompanied by a tapping noise @ idle which is the piston "rocking" @ TDC.
I guess the only way to identify your oil usage is either a leak down test or a borascope - assuming that you don't have a big oil leak which would be very obvious.
Have a look at
Porsche Engine Repair and Service Specialists and see Engine Guide for some interesting - if long & detailed info.