The PX-712A will burn some 8x DVD+R media at 12x as has been mentioned by Zevia, this does not necessarily mean that the results are as good as when burning 16x certified media at 12x however. It also doesn't mean that the burn quality is necessarily as good when burning at 12x as when burning at the certified 8x speed.
I have had both good and bad 12x burns with 8x media on my PX-712A, such as Verbatim DataLifePlus 8x DVD+R (MCC 003) and Fujifilm 8x DVD+R Made in Japan (YUDEN000 T02), but burns on Verbatim DataLifePlus 16x DVD+R (MCC 004) have been the best so far when burning at 12x.
In the places I buy my blank media the price difference between 8x and 16x media has almost evaporated, so there's little to gain in buying 8x media these days.
Oh, and one last point: Even though the PX-712A can only burn at 12x it does so using P-CAV which makes it burn media in a shorter time at 12x than some other 16x burners do at 16x using Z-CLV, and faster than most other burners at 12x.