I put it in the Getrag MT-82 6-speed gearbox in my Mustang GT, per the recommendation of all the professional mechanics on the forums (/s). These gearboxes don't have a good reputation, being assembled in China, but also because of a crappy linkage design--if I've got this right, the shifter is bolted to the floor pan, not directly to the gearbox itself--and to plastic-tipped shift forks. There are lots of stories of 'blown' transmissions, although later versions, like the one in my Mustang, are said to be somewhat improved. Note some Mustang owners aren't known for being particularly careful with their equipment, but I generally treat mine well coming from the MOWOG world. TBH, I didn't notice much, if any, change in the behavior of the gearbox with the SSII; in my previous Mustang I put Royal Purple Synchromax in and I did think it improved smoothness somewhat and made 3rd gear a little less notchy (and some new Mustang owners have installed it).
I don't think you can go wrong with any name brand full synthetic in a gearbox, but I wouldn't expect a 'transformation.' I do think the MT-90 I put in both my Healeys improved shifting smoothness and OD engagement lag; but I can't imagine the SS making a huge improvement over the Redline (but let us know if it does). BG is supposedly a 'veteran owned' business, if that matters to you.