AoA,
OEM spark plugs for vitz-14 (1KR-FE) according to the owners manual (available online) is DENSO SK16HR11, SC16HR11, (and in one place K20HR-U11 also mentioned).
But these are not available in the market.
However, most threads on PWFs recommended IKH-20 for this engine. Similarly, denso website also recommend IKH-20.
Though there is not a big difference in heat rating of 16 and 20. Should I try using IKH-20, or continue finding the SK-16?