If I take it apart, I use a general waterproof grease inside. If it's the new set, I just squirt a mineral oil in the gap and on the spring. Either liquid or a spray like INOX.

Someone suggested a motorbike chain lube for the spring to me as well.

Mine are 27 years old too.

I've found the newest Felco 2's stick more than the older ones even though they are exactly the same (tolerances maybe). I run them slightly looser and lube them more where the old ones can go without lube for years.