The US oil embargo against Japan didn't start until Summer. So this might have gone ahead before oil became an issue.
The Battle of Khalkhin Gol was not a lopsided victory for the Soviets as some imagine. Casualties on both sides were about equal. Tying down Siberian forces would have significant impact on the European front. It would mean the Soviet counter-offensive at Moscow in the winter of 41-42 would unlikely succeed. in OTL fresh Siberian troops were the backbone of that operation. That would give the Wehrmacht a window to capture Moscow.
While the
OIL embargo did not begin until after June, the U.S. had already begun to strangle the Japanese economy with embargos on Iron & Steel by the summer of '41. FDR was hesitant to cut off the oil tap as he believed (quite correctly) that it would mean war. There was no question, on either side, that the Japanese were at the mercy of the United States where her oil supply was concerned. That the embargo didn't start in June doesn't mean that both sides weren't sure that it was coming.
Khalkhin Gol casualties
may have been even AS A CAMPAIGN (actually, this is something that is still open to question, while Soviet records have been reviews so a reasonably accurate picture if losses is available, the same is not true for IJA records & loss estimates for the IJA range as high as 60,000, with the true KIA figure probably being a 15-25% of the high number), not that casualties are any way to judge victory or defeat in a military sense. The Red Army stopped the Japanese offensive cold; when the Soviets counterattacked in September, in a forshadowing of the Eastern Front victories by both the Heer & Red Army, they pulled a double envelopment on the Japanese and annihilated the IJA 23rd Division. At the end of the day, the Red Army held the field of battle, destroyed a Japanese field army, and pushed Japan back across the Soviet border. That is a fairly serious drubbing.
The Far East Front was never "stripped" IOTL as part of the effort to defeat the Reich. That is one of the fallicies of the Great Patriotic War. There were LOTS of "Siberians" moved, but these veteran troops were replaced by troops from Central Asia (leavened by some of the veterans of Kalkhin Gol), leaving 19 DIVISIONS, 1,200 aircraft and 1,000 tanks facing the Kwantung Army. Given that the IJA had less than 400 tanks on the Asian mainland, and that Japanese armor was, at best, pitiful, this was an overwhelmingly potent defensive force.
It is also sometimes forgotten that Stalin had a source IN Toyko (Richard Sorge) that was able to tell Stalin, well in advance, of Japan's plans (at least until he was burned in late 1941). While Sorge was not always listened to, frex - he had the actual DATE for Barbarossa but Stalin didn't WANT to believe him, he WAS listened to when the subject was Japan. By the time Sorge was arrested the decision had been finalized to attempt the Southern Strategy with all the risks involved.