You should be safe with it, especially since you are mixing it with your existing fluid. Some guys have mentioned doing a flush/complete fluid transfer with full synthetic trans fluid and having slippage problems so drive it easy for a while so the pcm can adjust to this as it constantly monitors shift timing and should be able to compensate for it if your run into this but it will take a number of driving cycles to make a noticable change-and again that is only if you encounter this scenario.