I am in Oslo just now.
I was on the Newcastle to Stavangar DFDS ferry on the 4th May.
I got to Oslo in 11 hours!
Thats a hell of a long time, that should have only took 7.5 hours.
Now, as the Doc' said, just because the E6 is down on paper as a "main road" you really have to take that in the loosest sense of the word.
Seriously, I have seen 100's of UK "b" roads that are better than some of the Norwegian "main roads"
So just bear in mind when you plan your time.
The ferry leaves Newcastle, and the first stop is Stavangar, it's there for about 90 mins, then off to Haugesund, then off to Bergen.