I think your burner is a PX-708A.
Go to Plextor's website:
and look under "Support" There you will find all the manufacturer's who make "Recommended" and "Compatable" blank media for this burner.
I haven't burned at 8X yet, but the below media is recommended by most people here.
Fujifilm 8X DVD+R with "made in Japan" on the label or on the box.
If you check the ID code with a program that checks ID codes, it will be: