X2: protect against UV and ozone. If you can, light air pressure in the bag also delays gas-out of the lighter petro-chems.
A tire 40 years old may still be structurally what it was (cord and bead), but certainly the exterior skin is chemically different.
The hydrocarbons are volatile, and the sulfur oxidizes. How fast petros migrate out to the exterior, IDK. There's probably some difference between brands and intended use (mileage vs. traction) but a new tire should be entirely homogeneous.
I'll bet that a Goodyear/Dunlop person could tell an unsafe tire from a spectroscopic analysis. Just compare a 1mm deep punch biopsy against new for % petro and sulfur.