20 is one step colder than the 16s I generally run twenties in most of my Toyotas for the last say 35 years 16 is recommended it's kinda hot plug I've found living in the heat one step colder seems to work a lil better holding off pre ignition in 100 degree weather.
I emailed Denso and this is there reply below Could you please confirm if the 4711 IXEH20TT has superseded the SC16HR11 as the current recommended part for this vehicle? Hello Mark, Thank you for contacting us! The Denso 3499 SC16HR11 is the OEM part used for your 2013 Toyota Prius (Level 3). Now Denso 4711 IXEH20TT Iridium is an upgraded plug that can be used as an alternative. Best Regards, Murphy
That's exactly right in so many words. I run the 20s and all of my cars for the last like I said 30 years the plug recommended was a 16 for that 30 yrs too. With different prefixes and suffixes...