This isn't an OEM supply. MS is the company that makes it. Also, your above comment on 6 and 8 pin having the same number of current carrying pins is not technically true. On a 6pin, the center yellow wire is specified as not being used. Many PSU makers do connect it, but it is by no means a guaranteed thing. The additional grounds on an 8 pin also smooth out power delivery for higher current, which the 6pin does not have. And technically, ground wires DO carry current, just in the opposite direction. You can melt a ground wire just as easily as a hot.
Anyway, back to the MS PSU, the specs state it has a single 49.6A 12V rail, which is good. But its very strange that a 600W PSU only has a single 6pin PCIe power adapter. I personally would not use a 6 to 8 pin adapter for a GPU, you are just looking to have an issue.