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 Yep; (most) DVD-Rs and DVD+Rs have an organic dye layer that's used to store the data; the laser in the burner activates this dye layer and permanently discolors it. DVD-RWs are similar, but use a reversible dye layer. The dye layer is what rots easily, due to its organic nature.
Whereas 'pressed' DVDs (the ones you typically buy in a store) are literally stamped; they are physically deformed during manufacturing using a stamp (that's specific to the production run, but reused across many discs). That's also how they can be produced so quickly, despite burning at home taking forever in comparison. There's no organic material involved so the disc doesn't 'rot' as easily, although it can still be damaged by eg. UV light.
All Blu-rays use a non-organic dye layer, and so do M-DISC CDs/DVDs, and so they are much less susceptible to rot too.
@joepie91 Neat, I learned something new today!