we have M8 media in TS4500 IBM library and LTO8 drives. Work as expected.
I only had to select media type during media discovery process as ULTRIUM V8M.
Media have about 8,3 TB capacity (when writing compressed data) and media type "ULTRIUM V8M"
Please note that once the blank tape has been written as "LTO7" (6TB) capacity there is no way to reinitialize it to 8M (and vice-versa).
Given that fact and some requirements regarding drive and library firmware I tested the proces on single tape - including real capacity measurement.