I have had the timing advanced and retarded. When advanced the knock sensor will adjust it to the fuel so no gain is possible.

 

I forgot to say the power difference is massive. I'm sure it has double the power on good fuel as summer fuel. I can feel the difference within 5 or 10 miles of the gas station.

 

I've been all over the timing. I lose power when I retard it and when I advance it no change, the computer will set it where it wants due to the knock sensor. I can tell within 5 to 10 miles after leaving a gas stop if it was good winter fuel.