There were many reasons why Hitler invaded Russia. And one was to destroy the Bolsheviks, another to gain living space, and a third to destroy the 'unter-menschen' (Sub-Humans);Slavs, Jews and any other indesirable race. These were Hitlers reasons.
Tactical Reasons, are to gain more industrial base, oil fields and to capture technology, and equipment. The fuel argument cannot be held up for those arguing for or against. Oil was all over, and there were large oil fields in North Africa, he could have gone there to capture oil. There was no need to go Russia, however Russia was easier to get to and the Caucasus oil fields were large.
Now the Third Reichs fuel supply was going to be run off synthetic fuels, and alternative fuel supplies i.e Gas. This was said in 1934 and although the production was increased, and consumption of conventional oil was reduced from 1,920,000 metric tons of imported oil in 1935 to 1,382,620 metric tonnes in 1936 by 1939 synthetic fuels still didn't make up enough. Although by then it was at approx 700,000 metric tonnes a year.
Attacking in June 1941 for oil is far from the reason. I've said before, and that's the true reason. Surprise, Hitler needed to hit them before they had re-equipped their aging forces which was to be ready in Spring 1942.
Stalingrad was very important, for a start it's a city with a huge industrial base. It holds a large population, therefore a large garrison of the enemy. Which if left untouched would be a threat at your flank. It was right next to the Caucasus oil fields, so capturing it would give an effective garrison to cover the fields, and also a place to process and use the captured oil effciently, and most importantly ship it out quickly.