The coolant cap refers to not mixing anti-freeze types. Americans, Japanese, and Europeans have each had their own approach and used some slightly different chemistry in their vehicles. Volvo blue follows other European brands. There are some new formulations in today's cars (Ford has one for which there is not yet an aftermarket substitute). Mixing types may reduce corrosion protect or boiling point, but all are water based. Generally, a minor flush with clear water and refill with the proper mix of an acceptable product, even if not the specified product, is all that's usually needed.

Mixing a petroleum or water based fluid into the brake system; that is a COMPLETELY different story. It would require immediate flushing and very likely extensive replacement of components with soft sealing parts.

Many fluids are not only made for certain uses, but your car require that certain specifications are met or exceeded. Not all brake fluid, engine oil, etc is interchangeable based on the label name. In other words, you need to know the right type of fluids for your car, and pay attention to which filler cap is for which fluid.