A5/B5 doesn't have to be a synthetic, but in North America, that's generally what you're going to find. Honestly, if a dealer cannot be bothered with stocking the correct oil, I wouldn't be trusting the dealer to do an oil change. Then again, I don't trust the dealer to do an oil change in any event.
In fairness, that dealer has done well with ordinary API oil for years, so he is facing a changing landscape. On the other hand, it's not all that difficult. He's not suddenly got a Mercedes Benz sheet thrust in his hand. Most of the top 5w-30 synthetics are A5/B5 already, so it's not that difficult for the dealer, really. Buy 5w-30 synthetic that most of us have heard of and it'll meet specification. Claiming he's never seen an A5 oil doesn't help his case. It makes him look more foolish. He's never opened up an owner's manual or looked at an oil specification? They get wadded up if you don't use an OEM filter but they don't understand viscosity and ACEA A5 or read TSBs?