Were sanctions really effective?
Yes. Russian oil began to quickly go down in price in December. By the end of the month, the average price of Urals had been $50.4 per barrel (the drop in prices continued in January, too). Some of the oil was being sold at $37 per barrel. For the three months before that, a barrel of Urals was being sold for $20-25 more.
Prices of other grades of oil were going down, too: the average price of Brent was $10 lower in December compared to October. But the price of Urals dropped significantly harder: the difference between Urals and Brent surpassed $30, despite the fact that the gap stayed at the $22-24 level from August to November. Before the Russian invasion of Ukraine, Urals was only $1-3 cheaper than Brent.