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How tight does an oil filler cap have to be?

Not a life or death question – more like a ‘hmmmm… is a gasket needed because could oil vapors be escaping around the cap?’ kind of question.

I have an aftermarket oil filler cap that I got many years ago (because the original one broke) which has been working fine. It did not have a gasket.

It should have a gasket or an O-ring. Finger tight.