Transmission Drain and Fill: 2008 IS350

I am planning on doing a drain and fill service to my transmission and also replacing the gasket and filter. I am in the process of ordering the Toyota OEM transmission fluid from Amazon but I would like to use a possibly better transmission fluid. I was looking at AMSOIL OE Fuel-Efficient Synthetic Automatic Transmission Fluid which is approved for Toyota WS fluid cars. So my questions are should I just go with the Toyota OEM fluid or is AMSOIL a better alternatives? And my second question is if I can use AMSOIL can I do a drain and fill and mix the fluid? or is this a bad idea?