Welcome!
As posted, the SubSim forum is chock full of great information from enthusiasts who keep the game alive. Also there are numerous great YouTube videos for playing SH4. Search SH4 on YT and check
RockinRobbins 13's channel
As far as running out of fuel, I agree with setting the unlimited fuel option, and after you gain a comfort level with operating the boat, the sweet spot for long range ops is 1/3 power as the engines use progressively more fuel per mile.
When you remove the unlimited fuel training wheels, a good rule of thumb is to return to base (RTB) before the fuel level reaches 50% unless there is a refit port nearby. Running a productive mission and running out of fuel 15 miles from a resupply point or home port can ruin your day... don't ask how I know...

Good hunting, Kaleun!