Batguy - That Max was doing the whole thing selfishly, but I think that to him it was a fair trade. Yes, he needed gas, but as it was pointed out in the film, he could have easily taken that gas, fuel up the tanker and drive away. Seriously thinking about it, the tanker would be a better road weapon, a rolling fortress if you can scrap together enough metal to re-enforcing every inch of it. Yet, Max didn't. To me, that's a sign that though he started it as a selfish act, it brought the tanker back on good faith, and that because he completed the task, the reason behind whey he didn't drive it when the offer was made was that he thought to himself, "I've already helped these people and I've got my reward. Onto the next task." In the scenes leading up to him going to get the tanker, he seemed like he was making an effort to connect with them, but not fully connect with those people. Afterwards , when they had finally accepted him, he got afraid of caring for them and decided to run. It's like he still cared, putting up this false front of just being no different from the typical scavenger with the exception that he wanted all the fuel he could have for his car so that they wouldn't care about him when he left. It's like he wanted to make sure they didn't like him, to keep his distance from them, but he still wanted to make sure that they got what he needed. So,yes, it is a selfish act, but not the selfish act we think of it (it's he's trying to fulfil the psychological need to do the right thing, the thing that made him want to be a cop in the first place, that thing that makes a person want to be good).
But then again, I could be wrong with this interpretation, but it doesn't make sense that he would have interaction with the Feral Kid if he didn't want to be liked by at least one person in that group if it was all a selfish act.
Max being a little bit friendly with the feral kid for a few minutes doesn't exactly make him a team player. That cost him nothing.
He did a few scattered things to be friendly or help someone else during the course of the movie. But it was far outweighed by his overriding concern for himself. Almost everything he does, even the majority of the friendly/helpful stuff, has a self-interested angle if you look deeper at it.
As for the truck, IMO it's irrelevant whether Max's car or the truck would have made a better vehicle in that setting. I think it's clear enough that he preferred his car.
The truck might make a better weapon on paper, but there are lots of possible reasons why the decision might still be more blurry than that. Maybe gasoline was easier to find than diesel fuel. Maybe he didn't want to give up every other little worldly possession he still had on this earth, which were all over the inside of that car. Maybe he trusted the car's reliability more than an unknown vehicle. Etc.