If you read the box it usually say Up to 8x.
In my system inserting a 8X media allow me to burn it at 4x, 6x, 8x. If I insert a 4X media it offer me 2x and 4x (sometime 6x with some media)
So if I feel like I need a slow burn I use 4x, all these speed are normally in the media compatibility list, you may want to take a look. Some software may allow you to burn at 2.4x 8x branded media, but I am not sure it is wise to do so and I would be curious to see scan made like this compared to full speed.
I was not able to locate the media list for 108, here's 109...