shows low oil after driving

What is your question?

Had engine replace in my 1996 Isuzu Hombre in April 2009 and have had regular oil changes since. In the last month I have started having the truck show that my oil is low after I have driven say 35 to 50 miles (highway) and when I stop the truck sounds like a thrashing machine,which I figure may be the lifters but when I check the oil it shows that I am fine on oil and the oil is not burned or anything so my question is I was told that I may need to replace some kind of oil/fuel sensor,is this what I need and if so what exacatly called.