I just replied to a thread on a GM truck forum about fluid and filter changes. Dextron 3 has been replaced by Dextron 4. Just FYI, I have a 97 Silverado with 250,000+ miles. I just had the transmission remanufactured with performance parts. The trans shop owner said, if you change the fluid and filter at 40,000 miles, your fine. But if you have 100,000 miles or more, leave it alone. Trans fluid had A lot of cleaners in it, and fresh fluid tends to wash away any ware film that has built up on bans and clutches, so it happens a lot, that after a fluid and filter change, the trans starts slipping. Then your looking at a rebuild, or remanufacture of the transmission. This shop has been in business for 40 years. He does a lot of transmissions but its very rear to do a Toyota. They almost never brake.