Missmatched Plugs?

I got a new tacoma in 99 it had about 200mi on it, well now it has 45000 on it and running a little bad. Ck engine light came on, took it to dealer they said 02 sensor bank one sensor one so I replaced it. Ck engine light came back on and they said its running lean.
Anyway I just changed the plugs and I had 3 denso plugs on the drivers side and 3 ngk plugs on the other. Since I got a "NEW" truck and I know nobody has ever touched the plugs while I had it, how is that possible....

 

Yup.... that's exactly how my 96 was back when I owned it.

 

Great thank you, any ideas on code p0171 I know it meens its running lean, I have new plugs, and put in a new 02 sensor the one before the cat.
What should I do next??

 

Take a look at your mass air flow sensor, look if there is dirt on the element, check to see if your intake system is tight. If there's a leak between your maf and your intake, you won't have enough fuel for the amount of air that is going into your engine. The maf calculates x amount of air coming in so he sends y amount of fuel but before it goes into the engine, it gets air from another place if your tubing is cracked or whatever so now there is not enough fuel for the air that is going into the engine.