I had a fairly serious oil leak from the base of the oil cooler on my 98 Mazda 626. The thing is, that after removing the old gasket, I couldn't see anything obvious wrong with it in the sense of there not being any obvious tears, cracks or missing bits. The engine facing side of the gasket is kind of flattened / squished as can be seen in the picture. So I'm wondering exactly what the problem with the old gasket actually is. Here are some pictures:
There are a couple of things going on to make that gasket no longer work. One is that over time the gasket has deformed from its original shape so that it actually matches the void in the metal better. The other change is that the material itself become more brittle and less pliable.
The result of these two changes is that the level of pressure that the gasket presses against the surface is reduced, and as a result it is easier for the pressurized oil to push past the seal.
It is counter-intuitive that perfectly shaped gasket would leak while the one that is not as precise and must be squished into place holds better.