why does oil prices vary???

The worlds largest oil consumer is USA. Due to ecomomical downtrend in USA the consumption has decreased tremendously. So this increase and decrease taks place

It depends on a lot of factors such as: how many barrels OPEC does produce and sell, tax in each county, supply and demand, interest rates, money supply etc. For example if you remember 4 months ago a barrel was like $140 and today is $40. why? OPEC decreased amount of barrels supplied. Countries increased their money supply because of recessions and because of that increase, interest rates went down. People are spending less, there are less investments. That's your answer in few sentences. You can write a 20 page paper on that topic.