Here's the specs.

383
8.6:1 compression
procharger, p600 at 5 PSI (nothing radical)
efi
993 heads, stock valves, fresh valve job on heads
hydraulic lifters
stamped steel rockers
40 hours run time tops

Doing a compression test I found about 6 cylinders with 20-30 psi lower that the other two. I did a leakdown and found 6 of the exhaust valves were not sealing well (even with the rocker arms backed off).

Both the intake and exhaust valves were backcut. Using a marking compound after the valve job, there was good contact between the valves and seats. Upon diassembly of the heads, the exhaust seats look good, but the exhaust valves had a thin, spotty deposit on the face.

My questions are these:

Are the exhaust valves starting to burn up?

Do I need to run Stainless Steel valves?

I would open the boost charge pipe and look for oil there

No oil there, but there was a lot of blow by. Seems that 4 hypereutectic pistons got a little beat up with some detonation.

Before the damage, everything was running great and I checked timing at 35 hours. It was set at 8° BTDC initially but somehow wound up at 2° ATDC. So I reset the timing, heard some rattling and then bad blowby.

Upon further inspection, I found that the harmonic balancer ring slipped 10° since the rebuild so that explained the offset timing, and having about 45° advance with 5 psi boost explained the detonation.

I know what I did with the pistons, hence why the story was omitted.