Oil is a fairly fungible commodity. If there are some willing to meet their needs with Russian oil they will likely be able to get a better price since Russia has fewer customers. Which means they will buy as much oil at the better price as possible. Which means they are buying less oil from the sources that serve those that are boycotting Russian oil.
The invisible hand of the market gets around boycotts that are only partial for a fungible commodity.
There is, and has been, a lot of cheating with regard to oil. The Iranians were able to sidestep boycotts imposed on them by undercutting their competitors. Most of the world, with the exception of NATO and American allies in the Pacific Rim like Japan, most countries is not boycotting Russia.
The downside to Russia's invasion of Ukraine is not only driving Finland and Sweden to NATO, but essentially making Russia subservient to China. The proud Soviet Union of Stalin and his successors itself becomes a satellite of China Xi accomplishes what Ghengis Khan did without Chinese blood being shed.