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When I changed the spark plugs on my 08' 4Runner, I noticed it had alternating Denso and NGK sparks plugs in the engine. The manual has them both listed: Denso (K20HR-U11) and NGK (LFR6C-11). What is the purpose of installing two different brands? Does this practice add any mechanical benefit?

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    If it's an 08 then it's not the manufacturer, but whoever last changed the plugs. – GdD Jul 16 '18 at 12:15
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As far as I'm aware, there is no benefit to having a mixture of plugs, and in fact it may have a negative impact as one sort may burn slightly hotter than the other - though I very much doubt it would be enough to be noticable.

The reason for listing multiple brands in the manual is simply to allow you the option - if you can't get hold of one brand, there's an alternative. It may also be that it reduces cost in the factory - they will buy then in huge batches, as as the prices fluctuate slightly it would enable them to buy whichever is cheapest on any given day/week.

I suspect that the reason you've got a mixture in your car is simply down to a mistake - either in the factoy (not unheard of!) or by whoever last changed them...

I'd fit a full set of matching plugs, of whichever of the two brands is easier/cheaper to get in your local parts store...

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    And also JIT ie who has stock to be able to deliver at that time point. Most likely it was a « cheap » mechanic only doing the easiest two... – Solar Mike Jul 16 '18 at 11:24

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