You can safely add oil to bring the level back to normal if it is low, but that is only a temporary fix and not a substitute for a real oil change. Topping off oil mixes fresh oil with old, dirty oil that is already breaking down, so contaminants and sludge still remain inside your engine. A proper oil change completely drains the old oil and replaces the oil filter, which is important because the filter traps dirt and metal particles and can eventually become clogged. If you only keep adding oil and never change it, your engine will not be properly protected and may wear out much faster. Use topping off as a short-term solution and schedule a full oil and filter change as soon as you can.