Wear in gen 2-3 transaxle vs gen 1.

I just replaced my factory fill ATF at 22,000 miles and it was certainly dark, much darker compared to new WS fluid or even WS drained @ 30,000 miles from conventional AT in RAV4. I did a lot of searches on Prius ATF UOA and calculated iron PPM/1000 miles in factory fill fluids and average viscosity in all 3 generations of Prius. These are the results:

Gen1: 1.3-3 PPM/1000 miles; 5.2 cSt@100C
Gen2: 2.5-7 PPM/1000 miles; 4.8 cSt@100C
Gen3: 2.7-4 PPM/1000 miles; 4.9 cSt@100C

Now, this is a bit of comparing of apples to oranges as we have 3 different designs of transaxles and 2 different fluids (T IV in gen1), but it certainly looks like more wear in newer transaxles. Why would that be? I would point to 2 variables: lower viscosity ATF in Gen 2-3 and switch from mineral to synthetic base. Apparently, miner oils have higher pressure-viscosity coefficients and greater film thickness:

What You Need to Know When Selecting Gear Oils



My take is, there is a trade off between max fuel efficiency and max wear protection. Toyota obviously chooses max fuel efficiency in the Prius. Since one can buy a lot of fuel for the cost of new transaxle, I question whether that is being penny wise and pound foolish.

I will look for alternative ATFs that are more shear resistant and have better additive package compared to WS fluid. So far, we have few UOA with those alternative fluids to judge any wear improvements.

 

Update, I did find Gen 3 ATF UOA that showed 7PPM Fe/1000 miles. So, no difference between gen 2 and 3. Also used WS viscosity can be as low as 4.7 cSt@100C. It's very good news during winter time, but summer is a different story.

 

I posted a graph with Gen 2 results here: Transaxle Oil Analysis | Page 9 | PriusChat

I'll post a similar one for gen 3 if enough data points available.