I remember last year being asked about this for CD-Rs. My answer is the same now as it was then:
Almost any oil based markers will do. Oil doesn't penetrate/disolve/harm the label layer ontop.
Sharpies I don't believe are oil based, but I have used them in the past and had no problems with them, but CD-R pens are really cheap (you can actually buy a 3-4 pack for about $3-$6USD) so why not just buy them to be sure.
Then again, I also believe if you find something that works for you all the time, stick with it (ofcourse everything might change with enough time lol).