3G Alternators come with a pre-installed underdrive pully, or a pully that's smaller than the stock one, anyway. Compare them, you'll see. This could make the belt a little looser and could be causing your slipping problem. A slightly shorter belt should help. That, or your tensioner may be getting weak, but it's probably the new alt.'s pully.
As for the spray on the alt.... if the belt is mis-aligned on the pully, you should be able to just look at it and tell. If the belt is installed and has been cranked up and run, look to see where the belt is sitting on the pully. If it's sitting in the middle where it's supposed to be, then it's fine. If it's sitting off to the side and is rubbing something, then you may have to use some small washers to space it out the where it needs to be. Maybe new 3G alt's come with some kind of lube or oil or something that will spew out on first operation.... I don't know. I got mine used.
Good luck with it!