While the Balkans did net the Germans a reliable source of oil, attempting to squeeze an invasion of the Balkans and OperationBarbarossa into the same summer was a gigantic mistake.
Others have pointed out the mistakes the Germans made regarding Barbarossa and their supplies and equipment, but had the operation kicked off on schedule (about a month and a half before it actually began), the Wehrmacht would have had a good chance of being in Moscow before the worst of the winter hit.