Why not just run the best known lube out there, Amsoil Series 2000 75W-90. They surpass all other gear lubes in overall specs, especially the non synthetic lubes. There is one test in perticular that runs for 250 hours at a certain temp. Mobil gear lube was tested against it and failed miserably. I don't know about Castrol syntorque, but I do think it's not a bad oil, just not as good as Ams. I would like to get my hands on some really good spec. comparisions and then have someone interprept them. Viper owners would want the oil that held up best in extreme pressure and that would resist thermal breakdown, as the Viper diff. tends to get really heated up when running hard. As far as the specs. I'm going to check in to things like this in the near future. The biggest gear and axle co. in Fort Worth will void the warranty on any rear end they rebuild, if synthetic lube is not put in them, regardless of what the owner customer thinks. When they leave his plant they have Amsoil Syn. in all of them. He said it cut his return rate down to nearly zero. This co. has been around a long time. It was many years before they found out what worked best.