2005 Mazda 6S GT 3.0L turned 200K & needs tune up. Looking for input on best plugs to use (brand, part number, gap) also P/N for upper plenum gaskets. Any/all input welcome.

 

LOL really??? Do a little work yourself and look up parts you like and then ask opinion. There is nothing you are asking form that you can't first the parts for first from top brands.

As for plug gap this is first subject to the end spark plug you choose.

Gaskets also can be found through several online retailers .

You have the basic information needed..

Year and month production
engine size
transmission type

What was the last services don in the last 4 years and what were they and at what miles?
Dude or Dudette youR profile does not even hint or state what part of the glob you reside on? HELPFULL!!!