Introduction
We have two baseline samples, Amsoil ATF and Toyota Type T, and
four vehicle samples:
- 03 Prius, 1st change, 53,000 miles (Type T)
- 03 Prius, 2nd change, 7,000 miles (Amsoil)
- 02 Prius, 2nd change, 40,000 miles (Type T) - c/o Woodson Moore
- 01 Prius, 1st change, 44,000 miles (Type T) - c/o Jerry Jorgenson
- 01 Prius, 2nd change, 20,000 miles (Type T) - c/o Doug Schaefer
This limited sample set is enough to project the transaxle oil change
interval and give some insight to what happens as the oil ages.
I can not endorse using Amsoil ATF in the NHW11 transmission due to
excessive copper contamination found in the oil sample tests.
Amsoil has twice the calcium and phosphorus additives of the Toyota
OEM Type T-IV and Type WS oils and the results are consistent with
a chemical reaction with copper.
Amsoil ATF retained viscosity in service better than the Toyota Type T-IV
but the risk to the copper stator windings are too great.
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.
Additives
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.