We have two baseline samples, Amsoil ATF and Toyota Type T, and four vehicle samples: This limited sample set is enough to project the transaxle oil change interval and give some insight to what happens as the oil ages.

Viscosity Life

The PdMA oil testing service recommends that oil be changed when there is a 15% decrease in the viscosity. As shown by the following graph, Amsoil ATF appears to be good for 12-17,000 miles:

This test is in progress and we anticipate another oil change in the fall. However, Doug's sample pretty well nails 15,000 miles as the likly useful life of Type T oil.

Wear Metals

There are two characteristics of interest: (1) age related wear particles and (2) usage related wear. As the following charts show, it appears copper and aluminum are usage wear particles. In contrast, silicon, iron and to a lessor extent, nickle, sodium, and tin appear to be age related, some form of chemical leaching.

Unfortunately, the 2nd change oil did not occur after dropping the pan and wiping down all exposed surfaces. Solution wear elements would easily 'rise from the sludge' not cleaned out in the first change. However, the current Amsoil test, the 3d change, was done after wiping down the pan and all exposed surfaces.


Transmission oil includes additives to protect the metal surfaces and control other aspects of the oil. Over time, these are consumed:
We don't know enough about them to state anything definitive. However, the increase in barium and magnesium from the inital oil changes are curious.