Goldshield, since you appear to scan with a BenQ, set the scan speed to 8x, as that is the normal speed BenQ expects to scan with. Rescan those and see if the results change, but scan all future discs at 8x.
Actually, the Imation dvd-r 8x (Fujifilm03) results depend on the burner. Look at this link to see that possibly that brand of media (FujiFilm03) is not a good choice for your BenQ1620.
Remember that the mid (this tells you who actually made the DVD–like FujiFilm03) is the most important identification to use, as the disk can say “Imation” or “Maxell,” but neither company actually made it. Always put in a blank DVD, and use CD-Speed to identify the mid. This way, you can return them to the store for exchange if they are not the ones you wanted, because you will not have burned any.
Better choices would be Verbatim 8x (MCC003), Fuji DVD+R 8x (yuden000t02-00), Sony DVD+R 8x (both Made in Japan on the side label), any OEM Taiyo Yuden (like TYG02)… Read through the “Blank Media” threads, but especially the “Post your BenQ DW1620 media here” thread (here’s the link for it, but at the last page), where they burn all different kinds of mids with mostly good results. Take note of the speeds and everything they do, if they use WOPC on/off, if they burn at the DVD’s rated speed or over it, if they used MCSE (the Speed Patch) and what write strategy they used…
Also, in the future, it would be a great idea to tell us what program you used to write the data to your DVDs. By the way, your first two burns at 4x with the Imation are ok/acceptable quality, so you might be able to burn the rest with the same quality at 4x. To make sure the DVD is readable, be sure to run a Read Transfer test on each DVD, and if the graph is smooth (no big spikes), then your DVD is readable and you should have no problem playing it in your DVD player or using it for data storage.
By the way, welcome to CDFreaks! I hope you enjoy your time here and find it as informative as I have.