I have finally had the chance to test DVD+R, DVD-R, dual-layer DVD+R, and dual-layer DVD-R in my Cube's original Matsushita SR-8186 DVD-ROM drive. This drive is using the Y213 firmware, which is a custom region free firmware, but otherwise it is the same as the usual SR-8186 firmware. Here are my results:
DVD-R DL: Does not work*
DVD+R: Does not work
DVD+R bitset: Works
DVD+R DL: Does not work
DVD+R DL bitset: Works
* I have not tested many dual-layer DVD-R discs. So far I have tested only one brand, but it is readable in my other machines, including my 5 year-old Panasonic RP91 DVD player.
The reason the above information may be important is the issue of +R and dual-layer recordable DVD media. Most drives work with single-layer DVD-R natively, but many older drives won't work with single-layer DVD+R or dual-layer DVD media of any kind, when the media's book type is properly set. However, one can get both single and dual-layer DVD+R discs to work on these drives simply by changing their book type.
How does one set the +R book type to DVD-ROM? Unfortunately, only specific burners support this feature. Most will only set +R discs as +R, because that is what they are supposed to do. However, some will allow the user to choose which setting to use, and a few drives just default to the DVD-ROM setting for maximum compatibility. Since my iMac's and MacBook's Pioneer burners don't support bitsetting, I keep an external Firewire bitsetting drive around. It is a Pioneer DVR-110D flashed with a Buffalo bitsetting firmware.
If you need some further information on the various DVD formats, please check out my DVD recordable FAQ. I have not updated it in three years so it completely ignores dual-layer recordable media, but some of it still may be of help to some of you as a basic DVD primer. For more in-depth and up-to-date information, you should also check out Jim Taylor's DVD FAQ.