oil light and buzzer on vw t4 1.9 td 2001?

3 weeks ago oil light 3 buzzes once only, had oil/fiter change shortly after, no prob till 8/1/11 after aprox 80 mile of motorway came on to slip road light buzzer as before, on 3 futher times in 20 miles. next day on once in 20 miles to motorway, not once 80 miles of motorway came on on slip road, twice in 5 miles, stopped for 15 mins did not come on again remaining 15 miles home, any ideas