Generally I would trust military surplus mags, more than commercial made mags for long term load.
I load only 25 rounds into a 30rd mag for storage, so far no problems when I've used the mags..
On another note, shot my enheirted M1 Carbine, one of the mags finally failed. My dad had them loaded w/ ammo since the 60's, but I've shot it quit a few times in the last year. Been looking for the mag spring but can't find one for 30rd mags...