4x CLV = 60/4 = 15 minutes.
2.4x CLV = 60/2.4 = 25 minutes.
So it's most likely a 4x writer
Your best choice may be 8x Taiyo Yuden DVD+/-R media in this case, because it might not support recent 16x media as cd pirate already mentioned (and maybe even has problems with some 8x media), and 8x Verbatim are already out of stock in some areas.
If you can update the firmware, do it... but I think the best would be a new burner.