5W20 engine oil is a multi-weight oil that is designed for use in winter to lubricate and protect the engines of smaller passenger vehicles. Check your owner's manual to ensure that you get the correct oil for your engine.
The 'W' stands for 'Winter', while the '5' indicates the viscosity of the oil, which tells you its resistance to flow. The higher the number, the more resistance. The '20' indicates the equivalent weight in accordance to regular oils, which tells you how well it will lubricate your engine. The higher the number, the thicker the oil and the more heat it takes to lubricate your engine.
While it won't cause terrible damage to your vehicle, using the wrong oil can have lasting effects. High quality oils like 5W20, when regular topped up, offer greater protection for your car against occurrences like sludge build-up and corrosion in the engine.