Huh, seems like self-written DVDs are more susceptible for disc rot than bought ones.
I found a bunch of old ones and a lot of them have dark spots, while none of the bought DVDs show any signs of disc rot yet.
@Vampire iirc non-blank dvd's are written with a different method than burning a blank one
@Ember I vaguely remember hearing that blanks for home use are different than blanks used in productions, but I don't know whether that's true.