[Nakulbhatt87]

I've heard various rumors about changing a car's tranny fluid over time. At one point, I was adamantly against changing the fluid because I was told by a mechanic that certain transmissions, if not properly taken care of will actually deteriorate in condition when you change the transmission fluid. This was something about metal shavings that were fused to the gears would dislodge and come out, after a trans flush, leaving your transmission in worse condition than before. However, recently I've been reading around and I am now thinking about the idea of doing a tranny flush. At this point i'd like to ask you guys and give you some background to my vehicles to decide whether it's safe to do the flush on all cars.
-I have three cars in question, the first car i'd like to flush the transmission on is a 3rd gen Maxima SE with the VE 5-spd set up. The trans feels a bit gooey and very ambiguous when its started up. Other than this I don't have many complaints about the car. I have no prior maintenance records and would like to do a simple drain and fill to make the gooeyness go away and bring the tranny back to life. The vehicle has no modifications and the miles are about 120k.
-The second car is a 4th gen Maxima SE 5-spd, with engine, suspension and tranny upgrades which includes a short throw shifter, light weight clutch and aftermarket flywheel. I bought this car from an auction and have no previous service records that indicate a tranny flush of any kind. I did not do these mods Im just using the car as a daily. The transmission makes a prominent "hissing" sound when in neutral and just cruising on the highways, it has approx 111k. I'd like to do a tranny flush and fill on this car to help with the noise and loosen up the gear box it self.
-The third car in question is a 2000 Infiniti I30 with an Auto tranny, the car has just about 130k and I have previous records that do not suggest any transmission services. This car has no performance mods.
If you all can please help me make the best decision for my babies i'd appreciate it. I don't race and I primarily plan on using Mobil1 Synthetic on the manuals and some Mobil1 Synthetic ATF on the I30. I drive my cars nearly all the time, Im looking for better over all performance, response and longevity of the vehicles life/parts. I would like to know if I should even think about touching the transmission based on the vehicle's history and it's current setup. I don't want to mess anything up by doing a transmission flush. I have already checked the stickies in this section and read about the ideal-non published intervals and which brands are available, and which viscosity and procedures on doing the flushes. I really appreciate your help.