.....Both Lycoming and Continental insist on supplying cork gaskets for
the
rocker covers, even though the far superior silicone type are
avaiable....

The OEM rocker box gaskets are really bad & I agree won't last anywhere
near 25 much less 12 years. I wonder what happens to those aftermarket
red silicon Real Gaskets that come back to Lycoming with cores? When
my OH time comes I'm gonna try to remember to hang onto my old ones.

Which brings to mind that any older overhauled Lycoming engines should
have all the rocker boxes removed on occasion anyway to check if the
free floating rocker arm shafts are wearing their way thru the cover.
Newer engines have a nylon insert to prevent that. I don't think it is
an AD but I recall a service letter or SDR on it. It is on my engine
watch list