Helping out a friend changing his spark plugs in a celica t sport, asked celica owners what to use? they have suggested IK20s, denso website is listing IK16s. Just wondering whether anyone on here with spark plug knowledge can tell me the difference, gonna be heat ranges e.g 6 or 7 but what would be better to use? Car is totally standard.

im asking you guys as i had a lot of help from people on here when ordering my NGKs for my S. Plus the celica forum is useless!

Cheers