Thanks very much to all of you for your feedback and advice. It turns
out the dealer didn't offer the engine flush this time. I don't know if
they discontinued it but the flyer is no longer visible on the service
counter.

On the other hand, they added a US$5.25 oil treatment on top of the oil
change (the service description said they used 5W-30, btw). I wonder
what could that be?