If you have a completely stock/OEM turbocharged vehicle, the addition of a Synchronic valve should not have an effect on the system's Air:Fuel Ratio so long as it is properly installed.
My idle AFR used to be richer with my old BOV, when I installed the Synchronic valve my idle went leaner. For example 13:1 to 13.5:1.
ANSWER: If your old valve was closed at idle, and you've now added the Synchronic valve, which is now open at idle, this could account for a leaner mixture because the open valve unloads the compressor in conditions where the throttle plate is essentially closed, such as idle and cruising. You can verify this by removing the vacuum signals to the back of the Synchronic valve, capping off the vacuum line and measuring your AFR.
ALTERNATIVE SOLUTION: You can adjust your pre-load on the Synchronic valve to close the valve more under idle and light throttle conditions. However, you need to keep in mind that adding more spring pre-load slows down the valve's reaction time, increasing the likelihood of producing compressor surge or flutter.
Under boost my AFR used to be richer with my old BOV, when I installed the Synchronic valve my idle went leaner. For example 12:1 to 13:1.
Is my BOV leaking? If your BOV was leaking and you have either an MAF or MAP sensor car, your AFR would actually go very rich if you have a BOV leak because the ECU is still injecting the same amount of fuel for less air mass entering the engine.
What is likely happening is that the Synchronic valve has eliminated a boost leak in your system or it allows the system to be more boost efficient than the valve you used to have in the system. If this is the case, the increase in efficiency tranlates into an increase in engine flow or cylinder pressure which means more horsepower that requires more fuel. You will need to add fuel to the system by tuning for more fuel enrichment.
You can verify this effect by eliminating the valve altogether and capping the mounting location temporarily with something that will be sealed and you will find that the same effect to be true.
ALTERNATIVE SOLUTION: If you have the ability to lower the operating boost pressure, you can reduce the boost by 1 psi and you will find that you are likely making the same amount of power at the lower boost level. This can be a solution until you can get a retune.