Dave, Everyone has their favorite gasket materials……
mine is the Permatex ULTRA BLACK #82180 "Maximum Oil Resistance" in a 3oz tube.
does not harden up in the cap like silicone ~ ~ ~ good stuff, in my opinion !!!
it does not affect the rubber spaghetti seal material in any way.
I have rebuilt probably half a dozen pre-1962 clam shell gear cases in the last couple of years
and this is the best sealer that I have found so far.
.