In all seriousness, cheapest would be DIY: oil and filter + for long term, a wrench, a pan, and a funnel. Takes an hour max. And it is actually really easy–I’m a mechanical idiot and can handle this task. Or call WSHS; I understand the shop class works on cars for free or cheap, and I’ve met the shop teacher and totally would trust him to verify the job is done correctly (way more than those chain places).