Set up business pages for Facebook, Instagram, Google, Yelp, etc. Post your services and contact info on all of them. Make at least a few posts a week with photos. Post in any local community forums. Ad space is typically cheap on restaurant tables or church pamphlets.
Talk to businesses that have fleet vehicles like construction, trades, delivery, etc. Pretty steady business right there just doing tires, fluid changes, and brakes.
“Free oil change” might turn away business if they assume it truly is free and find out they have to pay for materials. Maybe just set a “basic oil change” price and keep it fairly low.
Probably used thicker oil than recommended making the engine work a bit harder to pump it. Not sure it would affect it 5mpg but maybe, especially if it’s cold where you are.