This is an interesting topic! I know if I’d read it sooner I’d have quickly sided in with richardsmith in that a lawyer would love to hear about someone getting shafted by using a compatable, competitor product.
In cases where a specific blend is needed, I see a case here…………..until the patent dates and the competition clauses set in. I believe it’s illegal in this country to corner a market therefore a manufacturer’s demand for that brand would be in error. They could only demand that grade/property/blend of product be used. For one, not everything is available everywhere. Gianni nailed it. When it comes to marketing, the slightest of inconveniences often leads to poor sales and poor review.
I agree that it’s important to pay attention to what your particular motor needs, but you can’t limit those choices to only one source or brand without running into some trouble.
I haven’t read every warranty written but I know with my motorcycles/ATVs/sleds………..it’s always been stated a certain brand is recommended, but it was always a particular type of oil with this or that ingredient or property that was required.