Fuel line pressure gauge adapter, this?
 

FUEL LINE! (not oil like the title says) FUEL!!

Will this work?
Earl's 961966: OE Fuel Line EFI Quick Connect Fitting 3/8" Female to 3/8" Male | JEGS

Unplug fuel line at the pump, plugin in this, plug back in fuel line. Then I can go get a fuel pressure gauge.

https://cimg3.ibsrv.net/gimg/www.rx8...b5a53be691.jpg

Wrong size if you want to connect it to the OE fuel lines. We have 5/16" quick connects.

The part you want is the Aeromotive 15120 (this is what I bought and it works perfectly):

You will also need a 6AN plug since the 15120 comes with both 1/8 NPT and 6AN ports.

Is the one at the tank with the green clip 5/16" as well? It is bigger than the red and blue ones near the fuze box in the engine bay. Or did you just use one of those instead?

https://cimg6.ibsrv.net/gimg/www.rx8...feb132c08e.png
this is what i used under the hood by drv side strut tower

Nice. I'll install it using that part to the line in the engine bay instead of directly after the tank.

Blue or Red?

https://cimg4.ibsrv.net/gimg/www.rx8...df981e81c4.png

iirc its the red clip

What 200 says... Use the line with the red clip by the brake booster.