Manufacturers does not always use best stuff. They use what is good enough and not too expensive as they need to think about end product price to be competitive. Many manufacturers do use Iridium plugs as standard.
Iridium is the best and longest lasting as far as I know. LPG variation is supposed to be even tougher but not sure how suitable for petrol cars.
As for the make I normally go for NGK or Denso.
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Currently 8less
2011 Q7 S Line 3.0TDI, 2016 Tesla Model S 90D
8 history:
2006 A8 Sport 4.2TDI quattro SOLD,
1997 S8, reached end of life with gearbox failure
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