Valve seals or oil control rings? Which most likely?

More than likely the piston low tension oil control rings. A piston soak is simply a "hail mary"' because it doesn't address the actual issue. It's only going to be a stop-gap measure; if it works.
The other issue or future issue is that high oil consumption is going to kill the CAT; eventually.

I would start saving for another engine - NOT a used one. That's gambling and I don't consider tossing $3K+ at a used engine with KNOWN defects - probably ending up in the same place in a year or two. Or sell it.

Just my 2-cents

 

The Gen 2 engines do not have the low tension rings. The low tension rings started in 2010 Gen 3 engines.


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Nevertheless, the Gen 2 oil rings can still get stuck, leading to increased oil consumption.

If the valve seals are bad in a car typically there will be a puff of smoke at startup, when all the oil which dripped into the cylinder(s) while it was parked burns off.

 

Excessive oil consumption is common on old gen2 and gen3 engines. Gen2s may have had an advantage because they were 5k oil changes while gen3 were 10k by the book. Miss one gen2 and you are at 10k the next time while missing a gen3 puts you at 15k-20k.

The only way to resolve a quart every 1500 miles is with a ring job.

Valve seals are almost never a problem in modern cars but make sense if the engine is being torn down for rings.

Plugging your expensive catalytic converter is the real risk with excessive oil consumption assuming you or the previous owner did not run the engine dry of oil.

 

Rings are worn

 

Old rubber based valve seals used to harden and start leaking - NOT the new formulations.

Think of getting clay (in this example "sludge") in-between the rings and pistons and baking it. Chemical treatments will soften the surface layers; but you can't scrub between the rings and pistons or spray directly - that's why chemical treatments are a crap shoot. If that build-up isn't deeply baked in; that may work; but 200K miles of abuse/extended oil changes or missed oil changes doesn't solve that issue.

 

My son briefly had a 2007 Tacoma. He bought it from a friend whose father had passed away, and it was the father's work vehicle. It had sat outside for the better part of a year without being turned on or moved. After changing the fuel pump (which couldn't keep up under load) It worked well for a couple months and then literally as he was selling it it started smoking on start up. I don't know if the Prius has the same issue with valve guides going bad when the car isn't used for an extended period.

There are numerous reports that a Berryman Chemtool B12 piston soak can reduce oil consumption in a 2nd generation Prius. Perhaps in Boulder it is cool enough to do one now. I plan on doing my car but in S. California in the summer it is so hot every day that most of the solvent would just boil off (exiting the cylinder through open intake or exhaust valves, assuming it didn't flash off when it landed on a motor which was 120F or higher).