Finding Oil in Manchuria would not alter Imperial Japans strategic outlook.
True it may stop or delay a war with the Americans and Europeans, but it will not prevent the Japanese from Occupying French Indochina or engineering the Marco Polo bridge or Shanghai incident.
The 'advance south policy' came out of the fact that interdisciplinary rivalry between the navy and army came to ahead around 1936, the navy was incredibly frightened of the out of control military commanders in Manchuria getting Japan embroiled in a war between the Chinese and Russians.
Hence to steer the IJA away from the Russians they unilaterally began naval actions after the Marco Polo bridge incident to take Hannan island and Shanghai. These naval operations (and the supply to troops stationed here) is what brought about heavily oil usage, but at the time it wasn't 'critical'.
The trouble came when the IJN Occupied French Indochina, which sparked off a lot of anti-Japanese sentiment in Europe and America and what led to oil and Steel embargoes placed on Japan. This is when suddenly the American oil had been cut off, and so Japan had to go to war with America to return the oil supply for its navy to function and not leave loads of troops trapped in coastal regions of China.
At no point would gaining an oil supply in Manchuria have changed these outlooks.
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How well would have Imperial Japan faired against the Russians?
Firstly its not going to happen unless you butterfly away the IJN to a second rate department compared to the army.
For a scenario that Khalin Gol or similar escalates into full conflict, then the Japanese are fairly screwed since the majority of the army would have already been fighting the Chinese and the Russians would have been able to push deep into Manchuria (if they wanted to) before the Japanese regain the initiative.
If however there is no Chinese Marco Polo Bridge (or similar) then the question is a lot harder to answer.
In a war with only the Soviets the Japanese would have faired far better, they had comparative bombers, but while they lacked artillery batteries and tanks, this wasn't as huge an issue circa 1937. Japanese light infantry doctrine meant that although their units weren't well supported they could move fast and use the terrain more effectively.
This helps the Japanese on the defence because the Russian advance at this time will be slowed by having to bring up support equipment in very poor terrain.
Since this scenario precludes the IJA having preference we would likely quickly get an influx of light guns, and light bombers into the Japanese warmachine. The Japanese were fairly good at making artillery...god because the gun barrels were only just inside tolerances making them light for their calibres, they also had in general adopted the split and sprung carriages which allowed their guns to work as good as the day, particularly where roads did exsit.
The JAAF was also comparative to the European powers of the time, and also probally the most experienced airforce of the day.
Without the navy hogging material, the Japanese Airforce is going to see massive expansion since an aircraft can carry the firepower of the artillery without half the logistical issues.
This may prompt a form of 'heavy bomber development' in the medium to long term since obviously the Soviet east relies on the tarns-siberian railraod. Once the Japanese take that out of commission, the Soviets are fairly trapped. We may see instances like 'Stalingrad' but this time at Vladivostok, as the Japanese are able to isolate the city.
In the short term, the Soviets are favoured, but Japanese doctrine will favour them on the defence and in the long term aircraft development would allow the Japanese to strangle the eastern soviet armies. Thus it is on balance more likely to be a Japanese end victory...but fought across Manchuria without the Japanese being able to strongly push into the Stannovoys or Siberia. Thus the end result might be a return to the near status quo.
Of course the Chinese would likely backstab the Japanese and join with the Russians in the early war...in such a case Japan is lucky to hold onto Korea...but also that means that China goes communist (again).