The media may be quite different from one manufacturer to another. Verbatim ha been found to have less errors and be more durable. Apple itself uses Verbatim (the CDs and DVDs suppplied with Macs are Verbatim). Having said that, you have your own milleage.
All the Power Mac G5 Dual 2GHz, Power Mac G5 Dual 2.3GHz & Power Mac G5
Quad 2.5GHz can indeed burn to DVD+R DL as described in:
http://www.apple.com/powermac/specs.html
Optical drive 16x SuperDrive with double-layer support (DVD+R DL/DVD±RW/CD-RW). 16x SuperDrive with double-layer support (DVD+R DL/DVD±RW/CD-RW); writes DVD-R discs at up to 16x speed, writes DVD+R DL discs at up to 6x speed, reads DVDs at up to 16x speed, writes CD-R and CD-RW discs at up to 24x speed, reads CDs at up to 32x speed
But my question is can you burn brand new DVD-R DL (double layer) --not just DVD+R DL-- with such superdrive in the PowerMac G5?