As mentioned you probably will have to buy an oral measuring syringe. Another option would be an injectable syringe and to use that to measure (not inject.)
As to why the two different recommendations on volumes, it wasn't clear what the concentration is. Is it really 10 mg total in 30 mL? I doubt it, because then to get 20 mg/day you'd have to take 60 mL per day.
If it isn't that concentration, then you never said the concentration.
If it's 10 mg/mL, then you need 2 mL.
If it's 100 mg/mL, then you need 0.2 mL.
If it's for example 20 mg/mL, then you need 1 mL.
The volume needed to get 20 mg will depend on the concentration.