From me bruv who has been through this.....
"Firstly they’re two different engines. Gen 1 is the M96 (same as the gen 2 996) and M97 (the 3.8S) and on the Gen 2 997 they moved to the 9A1 (as also used in the gen 1 991’s).
Two elements - scoring and ovaling.
The cooling runs from the top down on the 1.2.3 (left bank when you stand at back of car) to bottom up on 4.5.6 (right bank). The causes the right bank to run hotter and makes it more prone to scoring and ovaling. The reason that the 3.8S is slightly more prone, but not immune, is the cylinder walls are thinner than the 3.6.
Combined with the above locasil can wear and break away from the surface making the scoring get progressively worse. The piston start to ‘tip’ which causes the edge to ‘tap’ the heads which is why you listen for the tapping noise.... as well as look for the left sooty exhaust. It’s the left exhaust tip that goes sooty because the exhaust crosses over - I.e the left tip is on the end of the right bank manifold.
If you ignore IMS bearing fear mongering the later 2000/2001 gen 1 996 3.4’s and the 986 3.2S have the most resilient M96 engines.
Incidentally there has been talk of the piston coating being an issue on 9A1 but I’ve not bothered to read in to it.
Basically, you either buy one and accept you may get a 11k bill or buy one that’s already had a rebuild. its still cheaper than buying a new car with high depreciation. Just ducking drive it and enjoy it FFS."