Keeping the correct level of frying oil is really important, but please do not use your 15 litre or 20 litre drum of frying oil to top up the oil level. Its heavy and more difficult to control.
Instead just keep handy this stainless steel, Frying Oil Pouring Jug. The large opening allows easy filling and the narrow exit ensures that you top up to just the right level. Light, easy, and safe.