Two years ago when I refurbished my CIS, I replaced that breather cap gasket.
Yeah, there
was an engine under all that gunk! This year when I swapped out my alternator, I found the same thing - oil/dirt buildup, just not 30 yrs worth. When I replaced that gasket, I installed it dry (cleaned up the parts well, of course). This time, I am thinking something like Curil-T will fix it (hopefully!). Out comes the engine tomorrow.
Ditto on the transmission shift plate cover....new gasket (installed dry) that subsequently leaked again! I replaced that one this year, with Curil-T, and so far it's not leaking.