Originally Posted by new

The transaxle oil is good to ~30k miles in the NHW11, 2001-03 Prius, and ~60K miles in the NHW20, 2004-current, before the viscosity drops below 15% of the OEM levels. The NHW11 uses Type T-IV and the NHW20 uses Type-WS. Amsoil ATF tests revealed excessive copper contamination, do not use.

In dusty areas, use a shorter change interval because there is no filter. The NHW11 has a pan that should be dropped, the magnet inspected and wiped clean. The NHW20 has no pan. There is about a 25% dilution from inaccessible drain oil and contaminants.

Originally Posted by original

Check transaxle oil early and replace if contaminated or discolored. Early results from an NHW11, Amsoil ATF experiment suggests an annual change or ~15,000 mi., will correspond to the worn out, 15% decrease in viscosity (31 cSt @40(C)). NHW11 uses 'Type T' and the NHW20 uses 'Type W'.