 Do you leave on a big trip with a half tank of gas in the car? No, you fill it up. Do you go into extreme heat with a half dirty filter? No, you put in another one. It's not worth it. Maybe an $8 or $10 filter or maybe more, but that's a lot less than a $200 service call later on. So I would encourage you and maybe ask that neighbor. Say, hey, do you have a good clean filter in your air conditioning system? It's going to be real hot next week.