Do you know what I love? Learning insider secrets. I’m not talking tabloid headlines or conspiracy theories, which I work hard to avoid, but solid, authentic, and reliable insider secrets. I’ve got one for you today, prompted by a question sent in by a faithful reader. Dear Mary: I recently purchased a late-model previously owned vehicle. The dealer tried to sell me a package that involves treating the leather seats. Because of the cost, I opted not to purchase the package. Do you know the type of treatment that car dealers use to treat leather seats? Is it even necessary to do this? The car is an expensive purchase for me, and I need to know how to take good care of the interior to make it last. Thank you for your very enjoyable column. I read it from top to bottom and always learn or find something I can …