That nasty, thick-n-sticky silicone grease is what the polyurethane manufacturers all recommend. By nature the black bushings have a little more graphite in them, but I don't think that automatically makes them "self-lubing" like some claim. I think you have to add even more before that becomes a real thing.
But I could be wrong.
Can't go wrong with that silicone stuff though. Nowadays it's available at most parts stores. Sometimes it's used for other things and just comes in tiny little packets, but you'll want a few of those. You can buy it in a small bucket too, but that might last you thirty years!
Paul