maintainace lights on

[R2D2]

Walt,

According to Toyota the WS ATF doesn't need to be changed. I personally am going to service mine regularly because I plan on keeping it for a long time. If you plan on keeping yours beyond 100k or I would service it. Toyota is the only official source of WS but other fluids are recommended for it including Valvoline Max Life, Amsoil, Redline, and Amalie.

Why?,

To address your question transmission oil should really only get "dirty" from wear particles and oxidation of the fluid itself (although other sources of dirt or water are possible). This would be most detrimental in that it could cause accelerated wear, but in severe cases oxidation and the subsequent thickening of the fluid could affect FE (though probably very little). I would worry mostly about the increased wear issue, however. If I owned a manual Yaris I would change the gear oil in the Yaris every 30k with a high quality fluid.