There are a lot of problems in this kind of plan. First off you don’t know really what are on these convoys. They could be men, tanks, food, ammunition, etc., you don’t know the exact cargo of each the ships. Even if you tried to find out there was oil on one, how would you attack the rest of the convoy? And what would you use for these piracy actions? The subs? You also now make the subs very vulnerable as they have to surface and attempt a boarding action against the cargo ships who would be armed. And good luck trying to use the surface fleet of the Kriegsmarine which was still devastated from their actions in Norway and would be no match against the Royal Navy or the US Navy combined. The convoy had escorts of destroyers armed to protect the convoy. Alongside planes and seaplanes who can be scrambled out or already on patrol and could engage the subs. And even if you somehow got past all of this and got a ship with oil on it, and the crew did not fight back while they took the oil, how would you transport it back? The subs can’t carry that much and the amount they could have carried wouldn’t be enough to fuel one Panzer battalion. Or would you try and hijack the ship? If the rest of the convoy isn’t dealt with they would fire on-the ship. Or it could be reported to other ships or planes and be sunk by them

The amount of oil you need is in the millions of metric tons per week to help with the fuel crisis. The actions of dozens subs maybe getting oil every week wouldn’t be enough and would risk the entire sub force to enemy fire. And the surface fleet of the weakened Kreigsmarine wouldn’t stand a chance against the full power of two worlds powers’ navies.