Beijing sees an opening to set tougher terms for buying Moscow's fuel.
Stockpiling may cloud outlook on prices.
Beijing tries to curb backdoor finance schemes through arm's-length corporations.
But observers suggest the shift reflects an expectation that numbers are unlikely to improve.
Beijing claims progress in energy efficiency suggesting it could help it meet five-year goal.
Russia promotes plans for a second line from the west that officials say could be agreed upon soon.