Drain Plug size for manual gearbox? M18 or M20

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Hey guys,
My 5-speed manual’s been leaking gearbox oil for a while now — probably the washer, but I want to replace the plug too. I’ve always reused the original one.

Does anyone know for sure if the plug size is?

M18 x 1.5
M20 x 1.5

If in doubt, I can just buy both, it's going to be one of them. Interestingly, my huge workshop manual doesn't specify this either.

Cheers!

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Update:

I found an older thread saying the part could be the "Doorman 090-075 bolt" which is a M14 x 1.5 part which makes sense as that matches the sump plug so it's going to be M14. Dunno why I was thinking the other sizes.