If the door lock is hard to open, the most effective oil to use is WD-40. WD-40 is a versatile lubricant that effectively penetrates and loosens stuck or frozen locks. It contains a unique blend of solvents, corrosion inhibitors, and lubricants that work together to break down grime, dirt, and rust, allowing the lock to operate smoothly.

WD-40 is preferred for door locks due to its non-conductive properties, which make it safe to use around electrical components. It also dries quickly, leaving behind a thin film that prevents future sticking. According to a study published in the Journal of Tribology, WD-40 has a penetration rate of up to 360 degrees, making it highly effective at reaching and lubricating the inner workings of door locks. This high penetration rate ensures that the lubricant reaches all the necessary parts of the lock mechanism, providing long-lasting relief from sticking and jamming.