I use mt-90 in my 8 (also aisin) and 75w90 for the diff
Like I said works great but needs changing every 30k, but with jack stands its a 40 min job (30 with hoist, both gb and df)
As for overflow, since this is a saddle tank, it needs to level out and sometimes purge gas (as in not liquid, gas) as you fill it, some stations have higher flowing nozzles. So make a habit to stop for a few seconds every 10 liters to allow the tank to 'burp' rather than forcing a lot of fluid in at once as causing it to spew back up.
Rx8 also has a saddle tank, please ask me how I figured this out lol (il give you a hint, had to go straight home and change/wash the jacket and jeans lol)
Also don't know if it has been mentioned, but when your low level light comes on, you have about 5/10 liters left (usually enough for 100km) this is how it is set up from factory so you don't stay stranded. Many ppl don't look at the gauge but only pay attention to the light coming on, not saying that for drivers of this car, just people in general
Sorry for the long ass posts, I don't post often enough to make shorter ones (like I used to) so now I cram as much in to one as I can. My post count was like 1100 at the start of this year or there about.. So not as active as I used to be
Cheers
Tony