I've been told there are three kinds of black beauties. The first kind are the ones with the big solder blob at the end. The blob is the little funnel they used to put the oil in, soldered shut. The just might be the worst capacitors ever made. The second kind is just paper, but probably less reliable than the wax/paper kind. The third kind is actually a mylar cap. Those are fine, and you can tell them by the color of the lettering if you know what the "right" color is, but I don't.
I wouldnt trust any oil caps very far... but I have no experience with the ones you describe. It might make an interesting little experement to test a few of them with a vtvm.
John