The problem with oils is that your car won't start shouting instantly if you pour a bad quality oil in it. But the engine will wither down gradually in a few months.
HOWEVER, in case of an excellent quality oil, your car will feel lighter, smoother, especially at idling, better gear ratios(incase of auto) instantly.
I have felt that difference with Petromin Oils. I tried it about 4 months ago, and now every car/motorcycle/generator in the family runs on Petromin.
Excellent value for money it is!!!!