I have a 96 960, and it is my understanding that the s90 is interchangeable with the 960. I got an extra transmission for my car when I bought it, and don't need it, want it?

I have a 96 960, after replacing many parts, it's started to idle rough. Code says cylinder 6 misfire. Plugs are new, coil pack is good. Clean AIC. Pressure in other 5 cylinders reads 180psi, cylinder 6 reads 10 psi. Could the valve itself be busted? Valve spring? If it was the o-ring , wouldn't the...