Interesting read. It's the CUSTOMER who chose to use synthetic, so regardless of what the "acting" manager decision is irrelevant.
If the dealership imperative to sell dino because of possible higher profit margins and pressure employees to sell dino irregardless of recommended oil by mfg or customer's decision, then it's your moral compass, financial viability, and your employable "worth" to decide whether you want to work for such company.
If sh1t hits the fan, ultimately the customer made the decision. End of story. Strong case for wrongful dismissal if you ask me