According to NGK, the "5" and "6" stand for heat range:I have a 2005 Odyssey EXL and I know that NGK's IZFR5K-11 fits the van,but I got shipped IZFR6K-11. Is it OK to use these? I really want to get the job done ASAP. Thanks guys
http://ngksparkplugs.com/docs/tech/design_symbols_plugs.pdf
"2" is "hot" and "11" is "cold".
Personally I don't think it is a huge issue and you can probably use IZFR6K-11. Anyway, only long-term use will tell.
For now, if you need to get it done, just make sure the you compare the old plugs against the new plug to be sure they are the same size, length etc.