It's all in the material you use and how/if you seal and tighten them.
Hate cork because it always ends up wet around the gaskets no matter what you do.
Also notice that stock iron heads are not flat but slightly raised if you look at them front to back.
In my own experience they are raised as much as 1/4 in the center and it makes sealing cast a PIA, but still can be done with thicker or multiple gaskets.
I use nothing but the black rubber on OE style covers.
Clean them up and straighten the sealing surface, then a 1/8 bead of RTV between the cover and gasket staying just on the inside of the bolt holes.
Then 1/8 bead of RTV down the center of the block side flattened and spread with a finger still going on the insides of the bolt holes.
Put it on and just lightly snug the bolts and let it sit for a few hours like that.
Then come back and tighten them down till you just see the rubber start to move around bolts and stop there.
I never have cover leaks doing it this way.
One the magnum with cast covers I used the Magnum tan silicone gaskets that are steel inside and still use a tiny bead of RTV on both side of those. (less RTV than OE covers, but I do them the same way)
BTW, I do consider myself better than the average bear at sealing things up, and my motor still looks like this months after the engine install.
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