Seals more than gaskets. You gotta keep them moist from rubbing fluid on them. If you don't they get dry and "tear" on a molecular level when you start rotating your shaft again.
Gaskets get pretty far gone and still work, a 23 year old Vette would need them regardless or not if it was operated regularly. I do plenty of valve cover gaskets on 15 year old beater cars and to a rule the old ones chip off like they're made of plastic.
Don't really know for sure but I can tell you my 2007 Corvette 24K just started weeping a little the last few years. I change the oil once a year. Just went from Mobil syn to valvoline synthetic high mileage to see what it does.