Let me see if I can help out.
Just because your drive says it is capable of 8X doesn't mean it will burn at 8X.
Depending on what you are using to try to burn with in the way of software, I suggest you burn at a slower Speed. like 2X. Apples Software is entry level and it works but many users find harder to use than the top of the line in burning software like Roxio's Toast 7.
Here is the explination on the different DVD's.
http://www.videohelp.com/dvd
Just bear one thing in mind.
When it comes to DVD Players, Formats, and Burners, of the hundreds of different manufactures, All are opting to be the Industry Standard, and it appears they insert there little chinks that almost force users to use there products exclusively.
There is always a backlash from these marketing ploys.
In a perfect world you would think any DVD would work in any DVD Player and Computer but sorry it is not so there are so many things that come into play. Format, Speed, Brand, Software used. Any one or combination can affect the process.
The problem is multi-dimensional.
First check in your particular profiler on EXACTLY what type(FORMAT) of DISK's your burner will burn.
Next please do not just randomly go out and buy a spool of 30 50 or 100 disc's because they are on sale.
There is a HUGE difference in the type format disc's you buy
There is also a HUGE difference in the BRAND you choose, some are junk.
There is also a HUGE Difference in the SPEED of the media you choose.
If any one or a combination of any of the 3 mentioned above can cause burn error's or no burn at all or spit the disk out.
So it is a good idea to do a little homework on your Optical Drive and see just exactly what it is capable of and what it will use for media.
(ONE LAST NOTE)
Many of the Media Manufactures are becomming aware that many of the Apple Burners are having trouble with there Media. Maxell has just come out with a brand new one that is. Maxell 16X DVD-R.
The key is read the packaging. this new media tells you that it will burn at any speed from 1X through 16X, and it works very well on the older Optical Drives. I also opt to use the individual that include the cases there seems to be much better quality, not like that of the spools.
Remember Apples Drives to date are much pickier than those used in the PC community.
Cheers Don