where is the 6sp trans fill plug?
 

I've got the Bentley manual, but it's showing the location of the 5sp fill plug (driver's side near the axle). Mine has nothing there. There is another plug at the rear of the transmission, driver's side, that's also a 10mm allen, just like the drain plug. Is this it?

Yep. That's it. What fluid are you using?

redline 75W90 NS...any thoughts?

Yup, the Bentley manual doesn't show it for the 6 speed. It's in the rear on the driver's side. Don't use Castrol synthetic. Personally I'd stick with Mobil 1, which is what Porsche uses and recommends.

I've read several places that Mobil specifically says Mobil 1 75W90 cannot be used.

Here is my experience with the 75W90NS:

http://986forum.com/forums/boxster-general-discussions/15062-redline-6-speed-cold-morning-shifting-test-results.html?highlight=redline+75W90NS

Results may not be the same for your car but I am sticking with stock fluid from here on in. I am not sure exactly what that stock fluid was. Part number was 4 000-043-304-71-SP TRANS OIL I wonder what the "SP" stands for. Swepco? Loren at Renntech would probably know.?