I was the one that opened the 5th Air... And it's working like a marvel now. Yes, 5wt oil works fine. Follow the instructions for the rebuild of the Singer 3Way... the rest is explained on the page at mtbr.com. Both shocks are surprisingly equal. Sorry, but I can read a bit of Italian, but can't write it not even to save my life. Stam... there are slightly different verisons of the 5th Air. Mine is a very early one. The 5-arm washer is a spacer. In my shock, if the 5-arm washer is not installed the oil pressure will just move the SPV valve, but you'll have no damping. I had the same problem as you... it lost all damping. But it was just that it got air in it and needed to be bleed. I also damaged the o-ring in the SPV Schrader!!! I just put some teflon tape and it's working great. I love my 5th Air... Now there's a Fox Float collecting dust... Next rebuild... a
Marzocchi Roco.