I just bought a 2016 Mazda CX-5, the manual says I should use Mazda's or Castrol's brand of 0w-20 motor oil.
Is there a reason why it says I should use this brand or is it just a marketing ploy to get me
to stick to their brand?
Of coarse they are going to push their own product, but you can use any brand you like. Just make sure the oil weight and type is correct for your climate.
Be careful here. It may effect the new car warranty. If something goes wrong with the engine the fact that you haven't been using Castrol Oil could void the warranty. HTH. - Jim