You should be replacing the plug and washer assembly every time you remove it, as the washer is captive and the threads do tend to stretch with overuse.
N90813202 is the correct part number, and has been for at least 15 years, across maybe 60% of all VAG variants.
I do come across the occasional 18mm head ones, but usually at the initial 15K service. These are probably what they use at the assembly plant.
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