It is entirely possible the problem is unrelated to previous Prius problems with rings.
We just changed the LH engine on our G550 Gulfstream jet. Before we changed the engine, we made darn sure there was not something simple causing the problem. The $30,000 worth of troubleshooting was able to ensure we were not making a $4,000,000 mistake.
The bottom line is to do the research, troubleshoot the best you can, including compression and leakdown compression checks, Prius chat forums and try a few fixes. That will give you a more clear picture as to the problem while only spending a little. With some luck the problem will be unrelated to rings.
I was simply making a guess based on past history.
Even so, "I feel" a more viscous oil would be part of the troubleshooting and would provide data. It's cheap harmless and easy.