The GM and DCX dealers get upwards of $20 a quart for this liquid gold. You can get a 16 gallon keg for a little less than 600 bucks from the Regional Castrol distributors. The best small quantity retail price I've ever found was here:
Is Castrol Syntorq and GM part # 12346190 the same thing? I have seen elsewhere the claim that GM part # 12346190 is Syntorq that is packaged for GM. I know that I cannot confirm nor reject the assertion.
According to CASTROL:
Message: Here is the response to the inquiry I sent to Castrol USA about where to get Castrol TAF-X in the USA: "Castrol TAF-X 75W-90 GL-4 gear oil is made in the U.K. and sold only in Europe. However, the equivalent in the U.S. is Syntorq LT. Having said that, Syntorq LT SAE 75W-85, API GL -4 is a premium high performance, synthetic gear lubricant for synchronized manual transmissions. The all new "clean slate" approach enables Syntorq LT to provide consistent performance and durability under the most severe operating conditions due to its unique polymer free formula. It has been designed primarily as a problem solving gear lubricant for manual transmissions to provide reduced gearshift effort at low temperatures. Syntorq LT is available through General Motors (Part #12346190) and Chrysler (Part #4637579) dealerships. Please contact your local dealership."
Actually Amsoil does make a more than suitable substitute. True it was not directly reverse engineered as Torque-Drive is (from TranSynd), but it works great - especially in the NV4500.