Type T --> 15,000 miles useful life

Hi folks,

Pending one more sample, we can pretty well nail the useful life of
Type T-IV fuild in the NHW11, 01-03 Prius transaxle as 15,000 miles.
Doug Schaefer's sample at 20,000 miles came in just over the 15%
viscosity loss signalling the need to change the oil:

http://hiwaay.net/~bzwilson/prius/pri_toil.html

His wear metals came in at half of the next highest, 40,000 mile,
sample. The straight-line between the 7,000 mile Amsoil and 20,000
mile Type T-IV intersects at 15,000 for both the 40C and 100C
viscosity lines.

I have one more test in progress that has another 5,000 miles to go.
This last test is checking my vent plug modification so I won't have
these results until later this year. But Doug's sample is enough to
justify sharing these results.

On my next change, I will be:

1) keeping the vent plug modification - avoids negative transaxle
pressure and I need the test results before reporting success or
failure. I may later add an oil desicant filter to further reduce
potential water infusion after I get my test results.

2) installing nipple Fumoto plugs - one in the drain and one in the
fill. These will be used for intermediate samples, oil level checking
and a "batch" oil filter test. In the batch filter test, I will
connect the nipples to a pump and filter and run it overnight. With
pre and post filter samples, we'll see if this can make an appreciable
reduction in oil particulates. It may also be a cleaner way to handle
NHW20 filter installation.

3) Type W - has a starting viscosity range at the ending range of the
type T. I suspect the Type T viscosity loss is mechanical, it is too
thick for transaxle use. The Toyota SAE paper on the changes between
the NHW11 and NHW20 cites use of an improved transaxle oil. I'm seeing
a 50F increase in transaxle oil temperature suggesting a significant
energy loss with Type T grade oil. I suspect Type W will improve
performance and reduce wear in the NHW11 transaxle.

I will be conducting an instrumented and documented test of the Type W
after my Amsoil ATF test is complete. However, I understand others are
already testing Type W. My expectation is the Type W should result in
a noticable mileage improvement of at least 3 MPG but we need
experimental confirmation.

I was asked why worn out Type T doesn't have simular performance to
new Type W? Certainly the hill rolling test showed new Amsoil ATF
(Type T equivalent) out performed used Type T.

The answer is the worn out Type T also carries a significant
particulate load and viscosity loss is an indirect measure of 'film
strength' for clean oil. Film strength is how much pressure must be
exerted for two surfaces to push through oil to make contact. This is
the 'slippery' factor of oil that along with viscosity, reduces
friction. But oil filled with particulates basicly has solids that
bridge the lubrication film. IT MAY BE POSSIBLE to filter used Type T
and get simular film strength of Type W but that is pure speculation.

So my recommendation for NHW11 owners (01-03 Prius) is:

1) change Type T transaxle oil every 15,000 miles until your transaxle
warrantee has expired. Be sure your first change drops the pan and
cleans out the gunk.

As for use of Type W transaxle oil in an NHW11 (01-03 Prius), that
experiment is in progress with one known sample and I will be joining
the experiment later this year. IF you plan to test Type W in an NHW11
transaxle, please consider doing the following:

1) get a Fumoto drain valve - so you can take intermediate samples for
testing. If you want to do the dual, nipple valve approach, fine by me.

2) drain, drop pan and clean - clean out all prior traces. If you have
less than 20,000 miles on your current Type T oil, please save a
sample in a clean container (dry polycarbonate or polyethelyne
container) and contact me about getting a sample of the 'gunk'.
Be sure to have tubing to drain the gallery to the rear.

3) perform early oil sample tests - we need samples at least every
15,000 miles of Type W oil in NHW11 transaxles. Basicly an annual oil
sample and test with replacement of the Type W sample to keep the oil
level constant.

BTW, we have one Type W sample and a baseline from an NHW20,
04 Prius. The one sample indicates the oil is good for 60,000 miles in
an NHW20. Until we get more samples from NHW20 transaxles, it is too
soon to draw any conclusions. But the first sample looks pretty good!

http://hiwaay.net/~bzwilson/prius/pri_T_cold.html

One last point, I see no evidence of bulk, transaxle oil over heating.
I have spent a significant amount of time at high speed on +100F
highway travel and the oil has never reached 85C, the ICE thermostat
temperature. This is with Amsoil ATF (Type T equivalent.) Limited hill
climbing testing, ~950 ft / ~290 m, shows no evidence of transaxle oil
heating. THERE MAYBE LOCAL HEATING in the bearings that I can not
measure but I have not seen a bulk oil heat problem, yet.

Bob Wilson