I recently used a mixture of Lucas M8 75w90 and Coastal conventional 75w90 in a Miata 5 speed gearbox. This particular unit has had a troublesome 3rd gear and this combo works extremely well. I also have used Amsoil MTG which is top notch stuff.
I recently read an article which suggests that EP products can be beneficial to engines as the rocker arms and some other components see high pressures.