From the U-Boat Commander's Handbook
Section II b 105 c
"Never delude yourself by assuming that it is right not to attack on the instant, or not to hold on the enemy with the utmost determination, because there may be reason to hope and believe that a better target will subsequently be found elsewhere. What you have got, you have got..."
In other words a tanker in the crosshairs is worth two in the gulf.
"Next question... if the attack was successful, and only 3 or 4 torpedoes remain aboard, would the skipper be expected to continue his patrol to the USA... or would he return to base... or would he be routed to a resupply boat somewhere."
3 or 4 torpedoes can mean 3 or 4 ships sunk. You should continue on your mission. Unless there are fuel concerns, a Kaluen that returns before reaching his assigned mission area with 3 or 4 torpedoes on board will be an ex-Kaluen scraping barnacles from old training subs.
A resupply would be nice, but even if you have no torpedoes you can still support the mission by sighting and reporting convoys and even using your deck gun if you can catch a loner.
The Submarine, sir, is an offensive weapon. Only food and fuel should limit the mission duration and contact with the enemy....and the crew can go hungry if necessary.
Good hunting to you.