Did you know that there are significant technical differences between DVD-R and DVD+R media? Neither did I. It turns out that DVD+R is a better bet if you are trying to use DVDs as an archival medium. (Not a great choice, but a choice nonetheless.)
Here’s an excellent article outlining the differences, including why DVD+R is a superior format, and a source for high-quality DVD+R media:
Here’s a link to the Wikipedia page for DVD+R. It gives a concise explanation of the differences between the DVD-R and DVD+R formats.