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This script will present a menu with three options when you run it:
Enter Odometer Reading: Prompts you to input your current odometer reading and calculates the remaining miles until your next oil change. Set Next Oil Change Mileage: Allows you to input the mileage at which your next oil change is due. Display Remaining Miles: calculates and displays the remaining miles without prompting for any new input.
You can add this script to your iPhone Shortcuts app. I use the shortcuts app to have it run whenever my phone connects to my car's bluetooth