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.
Stumbled on this site, some interesting claims
<a href="http://www.oilextreme.com/moil1.html" target="_blank">http://www.oilextreme.com/moil1.html</a>
Here's another similar type EP product
<a href="http://www.maxilube.com/" target="_blank">http://www.maxilube.com/</a>