Are you sure the plug on top is a fill plug? I seem to recall that there is a plug that should not be disturbed as it's for something else entirely?Top up via dipstick tube run the car for 1,000mls then do a "sump dump" and repeat two or three times to gradually refresh all the fluid rather than trying to do it all in one go.I do believe[although obviously you need to check with better sources]that you'd need JWS 3309 fluid or its equivalent.
Not really...

It looks like a fill plug, but in the youtube vid I watched, they filled it via the dipstick tube. So that's the way to go.

As too the fluid, the handbook says
"Only Volvo's gearbox oil part no 1161540-8. Do not mix with any other oil" 
Too be honest, I assumed that DexIII would do.....

Off to google for some research. I'll report back.
