P0011 Hyundai -engine sounds fine just came on. 160,000mils

Question

Everything I am reading about this code says it could be one of several things. What is the first thing to look at it? Car has 160,000. Checked oil and its full. Garage is not sure.. said timing belt then came back and said probably is an oil screen blockage 300.00 estimate for something unclear of? Do not want to hurt engine. No noise or acting odd. Suggestion where to begin. Starting with a new oil change tomorrow. Do not no when timing belt was changed; used car.