As Rone mentions, the crossing at Luvo/Songololo is good, it's the crossing that most vehicles take nowadays. The road south of the Matadi crossing(assuming that it hasn't been worked on in the last couple of years) is rough. I think Mateo and the biker guy that was riding with them took that road for the added adventure so if that is what you are seeking you can go that way. There was also a Danish guy on a sport bike that came through there and they made it but from there pictures it looked like the road gets somehwat chewed up in the wet season and the holes fill up with water. Otherwise, it's easy riding through Luvo.
There is a gas station in Mbanza Congo, roughly an hour south of the border. Although it can be spotty to get gas at the station. Once it runs out, the roadside sellers in Mbanza will have bottles to sell. I would say that your chances of getting good fuel is best there.
The road from Mbanza Congo to Nzeto on the coast has been rebuilt by the Chinese in the last few years so is good.
In Nzeto, there was no petrol station so the only fuel available was from a couple of roadside sellers. I met a couple of Americans in southern Angola that had purchased bad fuel somewhere between Nzeto and Luanda so when I came up I carried enough to get to Nzeto. If you've still got your magic green can, top up in Mbanza and you should have enough to get to Luanda or pretty close.
From Nzeto south, the road is sandy in parts in the northern part but rideable. Closer you get to Luanda the better it gets. On the outskirts of Luanda there was some new road construction that seemed to offer a quicker way in and out of town so keep your head up for that or ask around as you get closer.
As you probably know, in Luanda stay at the yacht club, free to camp.