Russia and Iran may have such and such common interests and especially when it comes to treatment of the West, but that does not stop the Russians from hurting the Iranian revenue coffers. 

According to oil market sources, Iran's crude oil exports to China have fallen sharply since the start of the war in Ukraine, with Beijing preferring to buy Russian oil barrels at a discounted price, leaving Iran “stuck” with nearly 40 million barrels of oil stored in tankers and looking for buyers. 

Western sanctions on Russia have led it to divert more oil to the east, and China is seizing the opportunity while reducing purchases of black gold from Iran (and Venezuela).