I never worry about when to use "which" or "that." They are becoming interchangeable.
That's one of the great things about English. It is ever-changing.
For example, hussy once meant a very valuable housewife and was not a derogatory term. I don't teach English and although I'm a writer, my main concern is getting the information presented in a clear and understandable manner.
Besides, oral language is less formal than written language, for the most part.
This info is just as important to me as the info I give Mother about the Porsche 911 993. 993 is vastly superior to the 911 996 because it has an air cooled engine, duh. She must know! Or others might think of her as dumb.