Version 02.00, August 7, 2007 - Warning to not use AMSOIL ATF.


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Prius cold weather performance falls off significantly:


Measure cold rolling speed change

During a cold week, we measured the coasting distance down a gentle hill close to our house. The initial temperature was recorded from the 'outside temperature' display and rolling stop locations recorded in the 'white' background markers. The Amsoil stopping locations were recorded in the 'green' markers:

Warmed up, the original transaxle fluid coasting reached a maximum speed of 15 mph and we braked going 13 mph. After switching to Amsoil, the maximum speed was 17 mph and braked going 16 mph. Bringing transaxle up to operating temperature makes a significant improvement in rolling drag for both lubricants. The following shows the energy relationships:

95,742 j - potential energy at top of hill

32,488 j - kinetic energy @16 mph, Amsoil
21,447 j - kinetic energy @13 mph, original oil
There was no attempt to adjust for drag from air, tire and stone-asphalt road surface. Tires were at 42/40 at beginning of tests but have not been measured or adjusted since. More cold weather measurements will be made when (if) the cold returns.


Amsoil ATF transmission fluid technical specifications indicate it has significantly improved, lower, cold temperature performance. It is listed as Type T-IV compatible, the standard for a Prius Classic (01-03). But there are risks when changing from the vendor's recommended, Type T-IV, fluid.

Expected Results

Due to the improved cold viscosity:
Risk: Electrical Characteristics
MG1 and MG2 use the transmission fluid for direct cooling and in the Prius Classic (01-03) that means exposure to voltages of ~273 VAC. The transmission fluid specification does not list the breakdown voltage or any electrical characteristics. The electrical characteristics of used Type T-IV fluid are also unknown and subject to considerable variability. So far, there have been no reports of internal shorts or arcing suggesting that used, Type T-IV fluid poses an electrical risk (In, we have a report of a 2001 Prius with 159k miles that had two coils burnt.) We have yet to find reports on Amsoil ATF use in a Prius.

The Prius II (04+) uses twice the internal voltage and a different fluid, "Type W". Again, we have no technical specifications for the fluid or any electrical characteristics. These can be measured but since we only have an 03 Prius, it remains an unanswered question.

Risk: Thermal Characteristics
The transaxle fluid transfers heat from MG1 and MG2 but we do not have the heat transport characteristics of either the Type T-IV or Amsoil ATF fluid. Our expectation is there is little risk of heat stressing either MG1 or MG2.
Risk: Use in CVT
Amsoil cautions against using their ATF oil in a Constant Velocity Transmission, specificly the Honda CVT. However the Prius CVT does not share the sliding cone, belt drive of the Honda. The Prius power split device, a motor generator and planetary gear, has more in common with a manual transmission than the Honda CVT or even an automatic.

How To Change Transaxle Lubricant

The following tools are needed in addition to the fluids: Be sure the vehicle is safely on a secure ramp - NOT A JACK!

There are three, 24 mm plugs. The two on the right are the transaxle drain and fill plugs. The one on the left is the inverter coolant drain. By accident, we drained and replaced the inverter coolant, which also has a replacement interval. This is much easier if both the inverter coolant and transaxle fluids are already available. Be sure to use the torque wrench to seat the plugs to avoid damaging the soft aluminum case.

When draining the transaxle fluid, have the clean sample bottle near and try to capture from the stream.

If this is your first transaxle oil change, get the transaxle gasket and throughly clean the pan and all exposed surfaces. There is no transaxle filter so the pan and magnet, which is loose, as well as all exposed surfaces need to be wiped clean with shop rags or paper towels. OPTIONAL: get the "o" ring and remove the take-up pipe and clean the inside like a barrel.

After draining the fluids, we have samples of the inverter coolant and the transaxle fluid. The transaxle fluid was dark, opaque and smelled of paraffin.


             ROBERT WILSON         TYPE T  53,000  01 Pri  02 Pri  20k-01   AMSOIL   7,000   TYPE W  61,000
                  Iron Fe  ppm          0      69     116      64      32        2      18        1     206
              Chromium Cr  ppm          0       2       2       2       1        0       1        0       2
            Molybdenum Mo  ppm          0       0       0       0       0        0       0        1       0
              Aluminum Al  ppm          1      21      17      16       3        0       3        1      56
                Copper Cu  ppm          0      75      67      62      15        0      28        0      21
                  Lead Pb  ppm          0       3       4       1       0        0       0        0       1
                   Tin Sn  ppm          0       2       8       7       1        0       4        0       3
                Silver Ag  ppm          0       0       0       0       0        0       0        0       0
                Nickel Ni  ppm          0       5      15      11       1        0       1        0       7
               Vanadium V  ppm          0       0       0       0       0        0       0        0       0
              Titanium Ti  ppm          0       1       2       0       0        0       0        0       0
             Manganese Mn  ppm          0       2       2       1       0        0       0        0       5
               Cadmium Cd  ppm          0       0       0       0       0        0       0        0       0
               Silicon Si  ppm          5     227     258      74      37        4      45        2     153
                Sodium Na  ppm          0       4      12       3       6        7       0        0       0
                  Boron B  ppm         81      43      46      64      60       48      41       59      27
             Magnesium Mg  ppm          1       1       2       3       1        0       3        0       0
               Calcium Ca  ppm        116     127     148     219     228      245     199      109     115
                Barium Ba  ppm          0      13       9       3       0        0       2        0      20
             Phosphorus P  ppm        269     255     279     247     354      415     320      245     227
                  Zinc Zn  ppm          2      38      26       0      16        2       9        0       8
                    Water  % vol      Nil     Nil     Nil     Nil     Nil      Nil     Nil      Nil     Nil
                   Solids  % vol      0.1    <0.1    <0.1    <0.1    <0.1     <0.1    <0.1     <0.1    <0.1
          Viscosity @ 40'C cSt       35.3    26.1    26.6    26.1    30.6     37.3    33.9     24.6    21.3
          Viscosity @100'C cSt        7.4     5.2     5.2     5.1     6.3      7.5     6.7      5.5     4.7
          Viscosity Index             183     133     129     126      63      174     159      171     144

PARTICLE COUNT (61,000 mi., NHW20)
                          4/ml              99999
                          6/ml               5466
                         14/ml                 25
                         20/ml                  9
                         30/ml                  2
                         40/ml                  0
              ISO Code  4  um                  24
              ISO Code  6  um                  20
              ISO Code 14  um                  13

PARTICLE COUNT (53,000 mi., NHW11)
                          4/ml              99999
                          6/ml               9054
                         14/ml                 77
                         20/ml                 15
                         30/ml                  1
                         40/ml                  0
              ISO Code  4  um                  24
              ISO Code  6  um                  20
              ISO Code 14  um                  13

Severity:  Severe
Recommended Action:
Check for sources of abrasives entry. 

Data Interpretation:
The iron,  aluminum,  and copper content have been flagged 
for observation. Abrasive contamination is suspected based 
on the silicon content. The viscosity of this sample does not 
correspond to the reference lube provided. Certain particle 
count values are higher than desired and have been flagged 
for observation.

Testing Service:
  PdMA Corporation
  5909-C Hampton Oaks Parkway
  Tampa, Florida 33610
  Phone: 813-621-6463
  Fax: 813-620-0206

The "TYPE T" is a baseline sample sent to establish what unused Toyota Type T-IV oil contains.

The "53,000" sample is the first transaxle oil change of an "03 Prius." This is the only sample to have a particle count

The "01 Pri" is Jerry Jorgenson's, 2001 Prius from Plano Texas. His sample came from an NHW11 with 44k miles.

The "02 Pri" is from Woodson Moore's, 2002 Prius from Denver Colorado. His sample is the second change from an NHW11 with 40k miles since the last change and over 100k miles total.

The "Type W" and "61,000" mile sample are from Dr. Denenberg's 2004 Prius from Connecticut. He provided both a Type W virgin and 61k used sample from his first change.

The "AMSOIL" is a baseline sample sent to establish what unused Amsoil ATF oil contains.

The "7,000" sample is the early, flush change of the Amsoil ATF fluid. This change was done at the sametime the transaxle vent plug was replaced by a neutral pressure vent tube to the air cleaner.

Oil Test Results

The interesting results are comparisons of the used to new OEM Type T-IV oil and the OEM Type T-IV to Amsoil. We also have Type W new and used oil and this is particularly interesting when compared to the Type T-IV.
Used to New Toyota Type W
The great news is the 100C and 40C viscosities were less than 15% under the virgin oil sample. The 61,000 mile change interval is right on the numbers for viscosity wear.

The particulate count was also lower than 53k sample from an NHW11. This suggests the wear pattern is much improved in the NHW20 models.

Type W to Type T-IV
The new Type W has starting viscosities in the range of the worn-out, used Type T-IV. It is also lower, significantly lower, suggesting lower rolling resistance than the Type T.

Toyota reports that between the NHW11 and NHW20 models, they used a lower viscosity transaxle oil and replaced one bushing with a bearing. These data suggests the changes should make a significant reduction in transaxle rolling resistance.

Used to New Toyota Type T-IV
Obviously the used oil has significantly lower viscosity. This strongly suggests severe wear and a loss of lubrication properties. Unfortunately, the oil testing service does not have a film strength test that would directly measure the librication property. But it is reasonable to assume that a substantial change in viscosity would go hand-in-hand with a change in film strength. However, the particulates in the oil would compromise the film librication and increase drag.
New Toyota Type T-IV to Amsoil ATF
The viscosity of Amsoil ATF is slightly higher than Type T-IV. In the operating temperature range, they are virtually identical which would lead to nearly identical operating temperature. The Amsoil viscosity index is lower which means a slower increase of viscosity with lower temperatures. Furthermore, the decrease in viscosity at higher temperatures should be less. To the extent that viscosity and the viscosity index is an indication of lubrication properties, Amsoil should be an improvment.
Old Type T-IV to Amsoil ATF
The hill rolling test confirms the new Amsoil has less drag than the old Type T-IV both at operating temperature and cold. But that has nothing to do with Amsoil performance compared to new Type T-IV. We do not know the relative film strength of new Type T-IV and Amsoil oil. The high partical count of the old oil would have compromised the lubricating film that seperates moving parts.
Viscosity of Oil Samples
The new oils have virtually the same viscosity profiles. However, the used transaxle oil had suffered significant degredation. It will take time to figure out if Amsoil is better able to handle the heat and mechanical stress that wore out the original.

One observation is the Amsoil cold temperature viscosity does not appear to differ significantly from the OEM Type T-IV. If anything, the Amsoil has slightly higher viscosity which suggests it should have higher flow drag than the OEM or used oil. But the hill rolling test confirmed the new, uncontaminated Amsoil has lower rolling drag than the lower viscosity, used oil it replaced.

Flush Oil Change

The 'flush' oil change was done after 7,000 miles included this time, removing and cleaning the pan. This was done after installing in a breather plug vent to the air filter. Impressions from this change were: The second sample came back less bad after 7,000 miles. The silicon level was 40 ppm, higher than desirable, but understandable because the first change did not include cleaning the pan. The oil testing service report that the 10% viscosity reduction was OK and they usually don't flag it until it reachs a 15% viscosity change. This suggests the oil should be good for 10-11,000 miles. We will find out this fall.


The plan is to follow this checklist:

Transaxle Temperature Probe

The thermister used has these temperature-resistance points:

The lowest resistance measured has been 511 ohms which puts this well below the 72.7 (C) / 169 (F) range. This suggests that ordinary driving even at high temperatures and speeds does not heat stress the oil.


We have started testing the effectiveness of an air shield to minimize excessive, cold air drag:
The initial observations: This new approach uses a pool toy, a "water noodle," to cover the two inlet slots built into the bumper cover:

Just cut a slit and the part will fit over the middle vane in the bumper opening.
To prevent it from 'working out,' I'm holding it with two strips of duct tape but eventually, I'll come up with some sort of latch.