Bit late to this thread, sorry.
The only way to burn DVDs at speed lower than 8x nowadays is to use an external slim burner.
Yes, I know the rule is that external drives are worse than the internal ones and the slim ones are even worse.
But in my own experience in the last several years the quality of the new internal drives has declined rapidly, probably due to manufacturers cost savings. At least where I live - south-eastern part of EU - right now there are only 3 internal models available - LiteOn, LG and the aforementioned Asus drw-24f1mt. I own the Lg and the Asus and they both look and feel very cheap (and they are cheap - 15 and 24 EUR respectively). Both are not great in burning DVDs, although Asus performs better, but again both are very far from the quality of the burners produced let's say 15 years ago.
But the burner that gives even better results is an external Transcend slim drive. Probably because it can write at 4x.
Anyways, sorry for the offtopic, just wanted to share my experience.
