i had bought a couple of 200 packs of Ritek G04's and G05's when i got my 1633S. i wasn't getting good results so i found this forum. after a little reading and experimentation (love that omnipatcher tool), i am now getting good, consistent burns.
for my 1633S, the G04's burn best with the G06 strategy at 6X. i burn the G05's with the stock strategy but at 6X. i definitely see a noticeable degradation when burning at 8X.
i always make my burns after a reboot, no other apps running, and using dvd decrypter.
i would suggest you try some different strategies (reset learning prior to each new strategy try). need to have a couple of burns to get calibrated (you may also get some type of i/o error on the first try of a new strategy, but try burning a second or third disc before you decide).
since the -R ritek's seem to be readily available, i am going to continue using them for now.