We had the same engine sludge problem in our 2000 Sienna. My experience as a mechanic told me something wasn't right. We were first turned own for warranty repair, told it was our fault, not enough oil changes and that the ones I did at home did not count.
Then a letter arrived from Toyota Canada saying they would do the repair if we could show we had one oil change done per year of ownership. We could and they repaired the engine.
The mechanics told me when I was inspecting the engine that these engines were cooking the oil so the solvents that keep the oil fluid and lubricating evaporate due to the high heat - hence the sludge.
I took picture of the disassembled engine and in my 25 plus years of experience had never seen anything so bad in 200,000 miles let alone 50,000 kilometers or 30,000 miles.
We received a second letter from Toyota warranty advising us that the warranty had been extended to 8 years from 5 on the engine repair unlimited mileage.
So why would Toyota repair the engine when they first told us we were negligent, lower the required frequency of one oil changes to one per year of ownership, and then extend the warranty to 8 years on this repair if there was not something fundamentally wrong with the design of the engine?
Almost all oil sold meets or exceeds standards for oil quality and performance etc. The dealership doing it is no different than the quick lube place or you doing it at home. It is not rocket science; hey it's not even complicated or technical to do.
Toyota Canada and I suspect Toyota USA use this, we didn't do it so it doesn't count as a smoke screen hoping their ignorant ill informed customers will go away.