I use both TDK, memorex and Imation brand richojpn media. Like i said in another post .some standalone dvd players have problems with plextor burning 8X to 4X media. I have also tried verbatim and have seen the same problem. No XBOX will play 8x burns on 4X media from plextor drive cause you get a dirty disk error. I have seen lots of people complaining about this on xbox forums.
Now it could be a limation of the 4X media who knows. ALL other burners only burn either 4X or 6X to richojpn the exception is TAIYO YUDEN most of the burners will burn that media at 8X. But we all know TAIYO YUDEN is considered the best media and that is why you pay such a premium for it.
I even went out and order a 5 pack of plextor media which is Richojpn and it would not play on my xbox at all either if burned at 8X.
Now i gave away the 8X copies i made to a friend who has a cheapo apex dvd player because they works fine in his player. Also all the 8X burned i made like 10 of them never got 1 coster they would play fine in my computer but they would skip in the samsung 931, Sony 770ns and i would get dirty disk errors on my xbox.
But same files burned on nec at 6X using same media plays fine in all my dvd players and even my xbox.
So i think it has something to do with 8X burning and amount of errors you get on the disk and how well the dvd player can deal with all the errors. And maybe the reason the nec seems to be more capatible is cause it only burning at 6X to same media and getting a much lower error count.