Returning to the ruins, Roland compared the fuel he brought with the fuel stored there. The diesel here is quite similar to the stored fuel over there, so out of instinct, he directly added that barrel of diesel to the generator tank and tried starting it. Although it could start normally, the fuel wasn't abundant, so it didn't last long before stopping.
Afterwards, they used the stored fuel to start the generator again, and charged the portal before starting to return. But this time they gained quite a lot, as he planned to bring more over after returning.
They also discovered that when the energy on both sides of the portal is present, the portal can actually remain continuously open, meaning next time Roland wouldn't need to carry a bunch of things himself; instead, he could place them on a small cart and push it over directly.
