EDIT: car is a 2000 Nissan Altima
When I first got my car it would shift hard from 1-2 and 2-3. My father changed fluid with aftermarket and problem went away but I don’t know what fluid he used. A few months later the torque converter was acting up because the thermostat was stuck open but I didn’t know why at the time. So I took the car to a transmission guy. He replaced the torque converter and said he rebuilt the transmission, replaced the solenoid pack and changed the fluid. I don’t know what kind of fluid. Since I got the car back it is shifting really hard from 1-2 and 2-3 again until I drive it for about 15 minutes. After 15 minutes it shifts smoother but not as smooth as it did before I took it to him. I’m thinking of changing the fluid. So if this time the problem is the fluid, would I need a thicker or thinner fluid than what he put in it?
If anyone is wondering, when the thermostat stuck open it wasn’t allowing the engine to reach full operating temperature which restricted the torque converter clutch operation. It happens with some cars. So in this case, all I had to do was replace the thermostat rather than messing with the transmission and torque converter