Hi,
I have run 3 significant tests of optical media, and can make very specific recommendations about the lifespan of the various types.
The reports of my tests are here:
http://mark-taylor.me.uk/?Optical_Media_Life_Expectancy
The quick summary points about long-term data storage are:-
1. Do not use standard recordable DVD (with purple dye) under any circumstances.
2. Use either BD-R or CD-R for good lifespan at low cost.
3. Use MDISC for best lifespan at higher cost.
4. Always keep optical discs in darkness (i.e. in an opaque box with a lid).