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I didn't know either but if you look at the maintenance schedule it says to change non-plat every 30k and all plugs at 60k. That is also echoed in the maint section on this site.

http://www.4x4wire.com/toyota/maintenance/tacoma_4runner_30k/maintenancep3.html#plugs

The only thing I can figure is that the plugs under the coil packs get a slightly better spark so perhaps they figure they could get away with non-plat on that bank?

Are you sure that you had 3 platinum and 3 non-platinum? Or did you just have the different brands (NGK/Denso)?

The maintence schedule is a generic version for all Toyotas of that year, that is why they list both Platinum and non-Platinum plug change schedules. And the link only seems to mention the non-platinum that the trucks use.

I think a lot of their cars had the platinum and their trucks had the non-platinum.

hupptoy

99 4Runner SR5 3.4 Auto 4x4, 32" LTX A/T, 154K
93 Toy x-cab 22RE 5-spd 4x4, 31" LTX A/T, 150K