You can guess all day about what could be wrong. Instead, why don't you run some diagnostic tests like a compression test or a cylinder leak down test? Those 2 test will tell you if your rings are worn causing excessive blowby.
As far as a blown head gasket, there is only one, 100% positive way to determine if its bad or not and that would be with a block test.
You can have white milky stuff in the oil cap and sometimes even the dipstick from condensation from running the motor in sub freezing temperatures and not letting it get up to full operating temperature (driving a solid 20 minutes) before turning it off.
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