The problem with that is they would have to import less of something else. Or they would have to reduce the intensity of their military build ups to reduce the demand for raw materials and mount an export drive to earn more foreign currency. AFAIK that is what the German government was actually doing in 1938-39.
In the case of Germany after 1937 a shortage of foreign currency was preventing them from importing the raw materials needed to run their economy at full capacity AFAIK. My thinking was that if the oil was discovered earlier a reduction in oil imports 1933-39 due to greater self-sufficiency would would save the German government foreign currency that they could use to buy other raw materials that they were short of such as iron ore so they could make more steel.
Also the money used to buy German oil instead of imported oil goes into the German economy and the German Treasury gets some of it back through extra tax revenue.
Furthermore when I've suggested that Germany should have put more effort into building up it's synthetic oil industry before the war I've been told two things.
- Germany didn't have the steel to build more synthetic oil plants.
- Several tons of coal were needed to produce one ton of synthetic oil and the German coal mines were already running at full capacity. Therefore it wasn't possible to produce any more synthetic oil before 1939 unless an ALT Treaty of Versailles allowed Germany to keep the Silesian coal mines that IOTL were awarded to Poland.
So if Germany could produce enough natural petroleum to eliminate the need to develop a synthetic oil industry that would release a lot of structural steel and construction workers by not building the plants and it would make millions of tons of coal available for other uses. One of those uses could be expanding the German iron and steel industry.
I've also read that Germany put a lot of effort into developing an industry to produce crude oil from shale, which was also a very inefficient way of producing oil. Again if the natural oil in Germany had been discovered and exploited in the 1930s there would have been no need for a shale oil industry that would release more resources for use elsewhere.