In The Matrix Reloaded, the Oracle encourages Neo to save Zion instead of saving Trinity. Assuming the Oracle truthfully wanted to help humanity, then why didn't she encourage him to save his love (Trinity) and not choose the door that leads him to rebooting the Matrix (saving Zion/humanity), but instead encourage him to go to the Machine City and talk to the Deus Ex Machina (as happens in the following film).
Neo did not have to choose between saving Zion or Trinity, he had to choose between following the normal path of The One (going through the right door, which leads to the Source) or attempt an alternate path (going through the left door, which leads back to the Matrix). If Neo chose the right door he would have no chance to save Trinity but he would certainly "save" Zion (by starting another cycle of the Matrix). If he chose the left door he would have a chance to save Trinity and Zion, though he'd be less likely to save Zion by choosing the left door.
The Oracle is not trying to convince Neo which choice to make because she makes it clear that Neo has already made his choice. The Oracle is trying to help Neo understand why he made his choice because no one can see past a choice they don't understand (not even the Oracle). Here is a transcript of the relevant part of Neo's conversation with the Oracle (emphasis added):
The Oracle: You've seen it, in your dreams, haven't you? The door made of light?
The Oracle: What happens when you go through the door?
Neo: I see Trinity, and something happens, something bad. She starts to fall, and then I wake up.
The Oracle: Do you see her die?
The Oracle: You have the sight now, Neo. You are looking at the world without time.
Neo: Then why can’t I see what happens to her?
The Oracle: We can never see past the choices we don’t understand.
Neo: Are you saying I have to choose whether Trinity lives or dies?
The Oracle: No. You’ve already made the choice, now you have to understand it.
Neo: No, I can’t do that. I won’t.
The Oracle: You have to.
The Oracle: Because you’re The One.
Neo: What if I can’t? What happens if I fail?
The Oracle: Then Zion will fall. Our time is up. Listen to me, Neo. You can save Zion if you reach The Source, but to do that you will need the Keymaker.
The Oracle can't see past Neo's choice (which is to return to the Matrix and save Trinity) because even he doesn't understand his choice. Therefore she doesn't actually know how it will all end and if Zion will be saved. She says this explicitly in The Matrix Revolutions:
Neo: You helped me to get here, but my question is why? Where does this go? Where does it end?
Oracle: I don’t know.
Neo: You don’t know or you won’t tell me?
Oracle: I told you before. No one can see beyond a choice they don’t understand, and I mean no one.
Neo: Can Zion be saved?
Oracle: I’m sorry, I don’t have the answer to that question, but if there’s an answer, there’s only one place you’re going to find it.
Oracle: You know where. And if you can’t find the answer, then I’m afraid there may be no tomorow for any of us.
Neo thinks his choice is between saving Zion vs. saving Trinity but that's not true. Again, the Oracle makes this clear in The Matrix Revolutions:
Neo: The Architect told me that if I didn’t return to the Source, Zion would be destroyed by midnight tonight.
Oracle: rolls eyes Please... You and I may not be able to see beyond our own choices, but that man can’t see past any choices.
If you look carefully at the Oracle's conversation with Neo in The Matrix Reloaded you'll notice that she's not really encouraging him to make a particular choice, she's (1) explaining to him that he has to make a choice (which is his choice, because he's The One), (2) helping him understand his choice so that both he and she can see past that choice, and (3) helping him reach the Architect (by telling him about the Keymaker, Merovingian, etc.) so he can execute his choice (i.e. walk through the left door).
This is a common theme we see in The Matrix trilogy, particularly when it comes to the Oracle. Sometimes being told literally what to do isn't the best way to accomplish the end goal. Consider the scene in the first movie, where Neo breaks the vase after being warned by the Oracle about it (23 seconds):
As the Oracle says, would he have broken the vase if he hadn't been told not to worry about the vase?
Shortly thereafter, she also tells him he's not the One (2:21, video above). And yet, as we find out in the end, he really is the One (3:18, video below):
So the reason she didn't tell him what to do is because telling him what to do isn't necessarily the best or right path towards actually accomplishing that goal. Like with reverse psychology, being told one thing may lead to the opposite choice being made. And for Neo, the important thing was that he had to be the one to make that choice.