Similar to this post: we wrote a blog about replacing your differential and transfer case fluid, specifically when it is required and when we recommend it. If you enjoy this post, that one is also a good one to check out.
For late model, 2001+ models Toyota, Lexus and Scion used two types of transmission fluids for their automatic and hybrid trans-axles, namely T-IV (Pronounced tee-four) and WS (short for World Standard).
T-IV initially replaced Dexron III as the transmission fluid of choice for Toyota and Lexus automatics starting in the early 90s. A conventional, non-synthetic fluid, T-IV requires replacement every 30k or 60k miles (depending on application) with inspection at every 15k interval. The replacement procedure is simply a drain and fill of the fluid held in the transmission oil pan requiring no special tools. T-IV is the transmission fluid for the 2001-2003 Prius, requiring replacement every 30k miles for this application. In these early Prii, the fluid acts to prevent corrosion of the hybrid drive windings internal to the hybrid trans-axle.
WS is now used in almost all automatic and hybrid applications since 2006. It is a fully synthetic fluid, requiring no replacement for the lifetime of the vehicle and only an inspection every 100k miles. For hybrid models CARspec recommends owners change their WS hybrid transaxle fluid every 100k miles as a safeguard against early hybrid drive failures.
CARspec stocks both T-IV and WS transmission fluids to meet the maintenance and repair needs of our guests.