90 Jetta low oil light/buzzer

1990 Jetta 1.8L auto :After driving 30 miles or so, on deceleration from interstate to off ramp, low oil light/buzzer comes on. Only happens during warm/hot weather - ok in winter. Oil level is ok and no engine noises. Does this sound like a low oil pressure sensor gone bad or incorrect grade of oil/filter? I'm running 5w-30 with a cheap-o filter - hear that most people run 10w-40 winter and 20w-50 summer with a factory oil filter or something made by Mann. Help me out! I also have some lifter noise - would the 10w-40/20w-50 solve the lifter noise as well as the oil light problem??

I have had the exact same problem in my '87, warm and cold weather. Going to a heavier grade oil has stopped the problem.