Assuming that the Germans captured the Russian oil fields in 1942 how?
Just for convenience, Im going to assume the Nazis not only captured the Caucasus, but also managed the capture the oil fields completely intact. The Nazis would still have lost the war. Quite simply, there was no way for the oil to get from the Caucasus to German refineries, and the Nazi position would have been horribly vulnerable to a de facto siege from British Persia and a pincer attack by the Soviets to cut off supply lines. Alright then, lets assume that the Nazis can not only magically transport the Caucasus oil to German refineries, but can defend said oilfields and supply lines from any kind of Allied attack, despite the fact that British-controlled Persia would have just bombed those oilfields like they did to Romanias. Yes, the Germans would get much more fuel, but would still not get enough to win. Only the Americans had something that would definitely change the course of the war: catalytic cracking, the first version of which appeared in 1937 and developed by Eugene Houdry. Catalytic cracking allows a greater yield of gasoline from a given barrel of oil, without as many problems as the previous thermal cracking processes (in particular, gumming fuel). While the Nazis would gain roughly 10% of the worlds then-current oil production with a Caucasus blow, that is nothing compared to the American share of nearly 70%. The Nazis would have never been able to beat American fuel production, and the Soviets would simply be supplied with American fuel. At that point the Soviet numerical advantage would have ended the European war the same way it did in real life, and Nazi Germany would have surrendered roughly September 1945, after nuclear bombs are dropped on Berlin. But lets turn the crazy meter up one (thousand) more notch(es) and assume the Nazis have all the fuel they need with zero supply security problems, and that the Soviets get zero material help from the British or Americans. The next major bottleneck for the Germans would have been manpower. Germany was already running into manpower issues as early as mid-1942. Even if the Nazis could supply all the fuel needed not just for themselves but for all their allies too, there wouldnt have been enough men to crew all the Nazis (and allies) tanks and planes and warships and trains and still have enough people left over to keep German factories producing all the material needed for a multi year war of attrition on the Eastern Front. Short of outright magic, the Nazis were never going to win WWII in any realistic way, Caucasus taken or not. End of story.