6.6 with converter drain. You should drain the converter, it holds the majority of the fluid, and is very easy. I would just stick with the dextron 2 or 3, I think the synthetic fluids are a hype and really just cost more money in the long run. You should in no way go longer than 30K on a transmission service, synthetic or not synthetic. FYI, ATF expans as it gets hot. If you do a complete service, add 4 qt's start it up add the other2.6 qt's and make sure to drive through the gears with little power for the first two complete shifts cycles.