Drain Plug size for manual gearbox? M18 or M20

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Hi guys
I assume it's M18 x 1.5, I've changed the fluid a few times over the years but never took much notice of the size. I've got a steady leak so I want to replace both the bolt and the washer, start fresh. Any insights would be welcome. Failing that I guess I just both both right

M18 x 1.5
M20 x 1.5