Q: Transmission will not go into gear when warmed up

Hello. From what you describe it would appear that your vehicle may be experiencing some sort of potentially serious transmission issue. If the transmission was having issues prior to mixing in the additive it may be possible that doing so exasperated the problems. Regardless of any additives, however, a transmission that does not shift properly can be caused by a few different things. A few common causes are low or dirty transmission fluid, or faulty shift solenoids. The transmission uses fluid pressure to shift the gears, and does so using electronic solenoids. If there is an issue with either of these the vehicle may experience problems shifting or transferring power to the wheels. If these are not the problem then it could be possible that the transmission is having internal faults, perhaps a worn torque converter, or transmission gears, inside of the transmission. As issues like these can sometimes be difficult to diagnose, if you are unsure, I would recommend having a professional technician, such as one from YourMechanic, visit your location to diagnose what may be causing your issues.