The upgrade of the 993 to hydraulic lifters also brought larger valves with smaller stems and guides, 8mm vs 9mm. This combined with soft bronze valve guides meant that guide wear is comparatively rapid on stock engines.
These engines need to have top-ends performed at around 100,000 miles for valve guide replacement and will begin to burn oil and smoke if left untreated. We have seen valves seize open in track motors, and while this is rare, the results are catastrophic.
The solution is simple though expensive. Update to harder bronze alloy valve guides. While you are at it re-seal the case through bolts, re-ring it, and maybe do head studs.