Despite being an old thread, maybe this will help others.
PS2 Slims, especially the latest models, have incompatibility with some games on the console itself, which is bizarre, but with OPL you can make a workaround where these games work even better than the DVD itself. I finished the Beyond Good and Evil game like this, with very few random sfx errors, no bother.
Then you will need to Enable the PS2 Logo function in the OPL this seems to somehow increase the compatibility and fix several errors, mainly the Boot, sound and crashes in FMVs. Also configure the IGR because if the game enters with bugs restart with IGR that at some point (usually on the second attempt) the game will work very well.
Important point, for PS2Logo to take effect, the console and the game must be in the same region NTSC with NTSC and PAL with PAL. Otherwise, the logo does not appear and the problem is not mitigated.
I tested Beyond Good and Evil with this method on a PS2 SLIM 90xxx.
The NTSC version was the one I finished and it went very well.. the PAL version didn't work well because it wasn't from the same region as the console, so I changed the input region from Beyond good and evil PAL to NTSC and it worked very well, better than the native NTSC version, I had no error with PAL version converted to NTSC and PS2 Logo enabled.
So I tested the game Critical Velocity where they say it doesn't run on PAL Slims consoles... I converted the game to PAL input with PS2Logo enabled and the game worked perfectly on a friend's PAL console.
I haven't tested other games said to be incompatible on Slims models but it seems that this method seems to have some effect, but via OPL.. I tested it on DVD and the game is really terrible.
Used OPL: Official Version 1.1.0 Final