Very bat service

I did go to buy Motorola on Saturday, I ask for Castrol 5w30, fully synthetic they didn't had, so the suggested liquid Molly, we ask if this is fully synthetic, they said it is, but when we read o the can it showed HC Synthetic what belongs to group lol what means it's not fully synthetic, so we went back to show them, they still argue that its fully synthetic, show I ask them just to show me on black and white that it's fully synthetic, but they couldn't (top tec 4600 5w30 liquid Molly) the showed me that they can't exchange oil, that is fine my question is why did they sold it to me as fully synthetic when it's not I believe them, as they work with it daily now I sit with this can of oul that I can do nothing