I assume, his burner is not able, to calibrate the laser-strength without depending on the "burning speed-laser-strength", defined by FW.
I said it several times,...
THX again, for a prove of my "guesses".
Slowest speed isn't the best, if the burner is not capable of do some calibration.
The organic layer in higher-speed-certified media reacts in a shorter time with the light-color-frequency of the laser.
So they are indeed certified for higher speeds and not for slow.
I also explained, how you can get the real certification-level.
8x should work mostly!
PS2 V7/DMS3 V2 (FW:2.4b7); Seagate Baracuda 200GB
PS2 V7/CC1.0 (FW:34 hacked v2 BM:2.1.6); Maxtor DiamondMAX9 PLUS 160GB
PS2 SCPH-30004R; NoMod+NoLaser
3xSony MC 8MB
MAX/Datel 16MB with Boot-CD
Custom FMCB 1.8b+ Beta-Build, my AIO 0.5, Sony&xRhino-Linux