Why would Soviet statistics suddenly become credible and trustworthy when it comes to grain and oil production?
The entire premise seems flawed.
It seems unlikely that the Soviets would manipulate their statistics in a way that makes them look politically "worse", i.e. if the data are false then imports are likely to be higher, and exports lower than reported which would tend to support the narrative.
There's also the issue that imports and exports involve external counterparties. Of course, you can still fudge your numbers but this will create inconsistencies vs the official statistics of other countries.
The Politburo wouldn’t want to manipulate it that way.
But thousands upon thousands of farm committees would definitely have tried to under declare how much grain they made, keeping some extra to barter as needed, and also to make it easier to hit targets next year.