I heard that adjusting the gas valve on my standard natural gas furnace will
make it more efficient.
By partially closing the gas valve reduces the flame. Although the furnace
would run longer, the flue temp is reduced meaning lower heat loss and
improvement in efficiency.

An analogy is like driving a car the same distance once fast and once slow.
When driving slower, it obviously will take longer, the engine is on longer,
yet fuel usage is less.

I have no way to verify if this is BS or not.