Nissan Repair: Transmission and engine, toyota tercel, bg products

Question
I have a general question .I have usually had my transmission serviced by simply dropping the pan putting in a new filter and adding new fluid,All the car repair places now seem to offer a "flush" or also called "fluid exchange" Many with BG products .I have 2 vehicles a 95 Sentra with 110,000 miles and a 89 Toyota Tercel with 140,000 miles they never had the tranny flushes done,Is it safe on an older car? Or should I just get the old type of service? I understand when you just drop the pan you don't get all the fluid out of the torque converter.Also should I use regular or synthetic fluid if I have never used the synthetic in either car?

Answer
Peter,

This is just me and my experience with automatic transmissions.  If you do not service them every 25,000 miles do not touch them other than keeping the level correct.  I had a Corvette with 125,000 miles on it and it ran fine until I changed the fluid.  A few weeks later it started slipping.  Now, the flush thing is one way to get all the old fluid out since what you do with a filter change is only a third of all the fluid in the transmission.  The complete flush gets all of it out and you are back to all clean fluid.  If you do regular changes you could probably get away with a BG flush.  If you have not change the fluid regularly then I would recommend against it.  Use regular Nissan fluid or equivalent.  Nissan actually does not recommend synthetic anything for their cars.  Check out the owners manual and somewhere in there they say that.