[bwilson4web]

BACKGROUND

The NHW11 Prius (2001-03) uses a different transaxle oil, Type T-IV, than the newest Prius that uses the Type WS. Type T-IV has viscosity characteristics similar to Amsoil ATF so it a natural to try it as a substitute for Type T-IV. Unfortunately, I didn't learn until later that Amsoil ATF has history attacking 'yellow metal' like bushing style bearings.

Testing of high mileage, used NHW11 oil revealed it wears down to viscosity ranges closer to Type WS. So rather than wear down Type T-IV, I decided to test Type WS and bypass the wear pattern.

RESULTS

The data suggests my NHW11 high Cu problem started with the Amsoil. Look at the rates:
[table="|"]Cu ppm/1k mi|sample
1.41 | 53k original Type T-IV
4.0 | 7k Amsoil
4.9 | 27k Amsoil
4.9 | 8k Type WS in NHW11 after Amsoil
2.7 | 27k Type WS in NHW11
... | ...
3.0 | 2k Type WS in NHW11 CA
2.0 | 325k Type T-IV Jesse
1.8 | 27k Type WS in NHW11 UT
1.5 | 44k Type T-IV [/table].
I'm still not ready to go back to Type T-IV because of the high starting viscosity. At this point a higher viscosity would likely put more stress on the bushings and remove the boron nitride on the surface. But it is obvious that going with Type WS has reduced the rate. It isn't as low as some of the others but I blame my Amsoil ATF test on the continued problem.

I did a hill roll-down test between the 53k oil and Amsoil ATF and there was a distinct reduction in rolling drag. But hindsight suggests it was the higher particulate load more than anything else.

Since we now have a ZVW30 (2010) Prius, I changed and tested the transaxle oil at 5k miles including a microscopic examination. We found wear material as would be expected with the initial change but also found a rate of viscosity loss suggesting that 15k miles may be when it wears out at a 15% loss. It will take a year before I change it again to see what the wear rate is after flushing the initial transaxle oil.

Bob Wilson