In which cases is it imperative to test the brakes?

You answered correctly. It is essential to test the brakes when you are driving a different vehicle because you are not yet familiar with its brake response. Also, if a vehicle has not been used for a longer period, the brakes could have defects or be less effective, for example, due to rust or sticking parts. Testing helps ensure your safety before driving. However, it is not necessary to test the brakes just because the engine oil is changed, as that does not affect the braking system. Remember to always check brakes in unfamiliar or long-unused vehicles to avoid accidents.