I had the same question myself...and decided to do a little experiment.
I looked up the specs on the LFR6AIX-11 plugs from NGK. All of the dimensions are the same (i.e. thread depth, seating distance, gap, etc.) The only other things to be concerned with are current and heat dissipation. Using NGK's charts, the plugs should be the same as the stock plugs. So I wrote to NGK...below is the reply:
"The Iridium IX LFR6AIX-11 should work fine, there is slightly less projection on the IX plug. As for why the truck came with a mix of Denso and NGK plugs, it a pretty good mystery and I wish I knew an answer. Thank you."
K. Zdral
Technical Support Representative
NGK Spark Plugs (U.S.A.), INC.
I have had them now for 10K miles. The results: No noticeable change in gas mileage or power.
I took them out at 15K to inspect them. No signs of abnormal wear or burning.
My main reason for changing them was after reading about te mix of plugs, Denso and NGKs. I was curious and pulled them to see for myself...I thought it was rather odd. I found the Densos to have had significant ceramic burning...at only 5000 miles. Rather then just switch them out with the stock NGKs...I decided to do the experiment.
So far so good. I am not "exactly" sure why they used the standard plugs along with the short service interval. The dealer still uses a 3K mile oil change for the car although even Toyota says 5K. Maybe they just want us to come to them more. All I know is the Iridiums for the V6 work well...and should last longer. I only paid $7.40/ea for them from Sparkplugs.com. If I go double the stock mileage on them...then they have paid for themselves.
