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With a six pot it will prob be 3 or more until you get a noticeable issue first thing will be a slight increase in fuel consumption as the cylinder with the glow plug out might not be clean burning as although it prob does have a glow the tip (business end of the plug) will be worn out. When I have my VW four pots it always was fine on 1 out but rough on two and dire on three. I always used to run Bosch plugs they were good on wear and were not bank breakingly expensive. It is advisable to replace all plugs at once so all cylinders are equal.
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