Hyundai Engine Sludge?

My Mother has a 2003 Hyundai Elantra. Recently she told me that the Oil light comes on when she turns and stops and then when she keeps driving it shuts off. Well, I opened the hood and checked the dipstick, nothing on it. The car isn't leaking so I opened the cap and the oil looks like it has mud in it and Its watery. So I put a quart in to see what that did, no avail. The problem has worsened and the light will come on more frequently since I checked it 2 days ago. Should I just do an oil change? She says the last time she had one was a long while ago. She seems to neglect her cars. :( Any ideas on a remedy for this situation? The car has about 110 K on it.