Keeping oxygen and water minimum is the most efficient procedure to extend the life of a transformer.
Utilities are procuring transformer with air cell, breather, thermo-siphon, however bad gasket allows ingress of moisture, abundant oxygen thus drastic reduction in life of transformer.
A good gasket hardly costs 0.3 % cost of the transformer. Bad gasket may consume 30 % life of a transformer. It is therefore prudent to include gasket testing in the quality assurance program.