Hi Neil,
Make sure you're looking at my updated post. Long story, but my DIY thread appears both in the DIY section and the GT3 forum itself. This is the one you want to view:
https://rennlist.com/forums/991-gt3/...il-change.html
The small green rubber O-ring isn't needed. Suncoast thinks it's needed when you spin on the filter, but it's not. The factory original had no ring and the workshop manual doesn't specify it. I told Suncoast that but apparently they're still including it in their kit for some reason. I didn't mention it in the DIY because I assumed Suncoast would eliminate it and I didn't want to cause confusion.
I only drained from the forward sump plug as that's what is shown in the workshop manual. If you wanted to pull both plugs you might get a tiny bit more oil out but I don't think it's necessary and removing the plugs in the tight space is kind of a PITA.
Good luck. Let me know if you have other questions.
Mike