Swapped the rear differential in my Mazda last month. Only major repair I’ve had on the car in 90k miles. Now I’m paranoid about getting at least the life of the tires out of it. Ran some Techron into the fuel 2x, tomorrow I’m going to pull out a bit of the engine oil and replace with crankcase stabilizer for a few hundred miles before my next oil change. The diff I put in was used and now has about a thousand miles on it so I will also charge the fluid once more with Royal Purple 75/85. Front transfer case too. Finally the throttle body. Should I bother changing the ATF fluid + filter if it’s never been done before? Heard it can create problems where there were none