Dell Precision T5400 (Late 2007) - Manual (Page 106)

106 Cleaning Your Computer
2 With a soft, lint-free cloth, gently wipe the bottom of the disk (the
unlabeled side) in a straight line from the center to the outer edge of the
disk.
For stubborn dirt, try using water or a diluted solution of water and mild
soap. You can also purchase commercial products that clean disks and
provide some protection from dust, fingerprints, and scratches. Cleaning
products for CDs are also safe to use on DVDs.