Actually, they do have to buy more oil than they produce. They have more than twice the refining capacity they have compared to production capacity.
They produce about 2.4 million barrels per day of liquids.
They refine about 5.3 million barrels per day.
http://www.exxonmobil.com/Corporate/Files/news_pubs_fo_2010.pdf
Page 28 & 76
Even if they refined 100% of what they produced, which they don’t, they still would have to buy even more than that amount to keep the refineries running.
It has nothing to do with their cash flow, but rather the business they own. Like most of the majors, rarely is the production of crude balanced with the refining quantities.
I did not post this thread to discuss the ins and outs of how Exon Mobile conducts it's business so this will by my last post on that subject.