Russia has an interest in US/Iranian antogonism not because of arms deals or oil prices, there's a much bigger strategic goal.
Iran as a US enemy prevents the creation of a stable Pakistan-Turkey/Israel pipeline, that would allow to tap oil resources from Turkmenistan, Pakistan and deliver them to US-controlled Middle East and from there - to Israel, Europe, bypassing Russia.
A Russia-free oil strategy would basically put to a halt all Russian attempts to increase their leverage in Eastern, Southern and Central European states.