How do I find a garage that specializes in my car brand?

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I drive a 2013 BMW 528i and I've learned the hard way that not every garage knows how to work on these cars properly. Last year I took it to a general shop for an oil change and they used the wrong type of oil. I didn't know until my friend who knows BMWs told me that my car needs a specific European spec oil. Now I'm paranoid about taking it anywhere. My car needs a new water pump and I want to find a shop that actually specializes in German cars, not just a place that says they can work on anything. A coworker told me to explore 800Sayara auto services on ProvenExpert because you can see what brands each garage specializes in based on customer reviews. I thought that was a good idea because I wanted to see proof that a garage had successfully worked on BMWs before, not just take their word for it. For those of you who drive European cars, how did you find your mechanic? Do you only go to specialists or have you had good luck with general garages too? What questions should I ask to make sure a shop actually knows what they're doing with a BMW? I'm tired of paying dealer prices but I also don't want to ruin my car by taking it to someone who doesn't understand it. Any advice from other BMW, Mercedes, or Audi owners would really help me make a good decision here.