Oil

So today I had to travel to Caernarfon for a meeting, what a hot day to be in the car then in a meeting. Anyway on the way home my oil light came on. I decided to stop to get Oil. To my surprise I had so many to choose from. The word that bothered me was synthetic. I wanted oil
But after speaking to a mechanic he said take it back you need fully synthetic..... When did we stop using oil. And how expensive £16.00