First ride after fork rebuild: brake funkiness + fork oil question

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I just rebuilt my GPz1100's forks and took it for its first shakedown
ride this morning. When I apply the brakes, EVERY time I apply the
brakes hard or soft, there's a "thunk" I can feel through the bars.
The harder the braking, the harder the thunk. At moderate to hard
braking there's another thunk when I let off the brakes. At first I
thought it was the forks because the first big hit was into a gutter
merging onto a street, but now I think it was because I applied the
brakes at that time.

I did torque everything to spec, and even pulled over to make sure the
caliper bolts were tight. It feels like it does when they're loose.
What else could be thunking around up there? Pads inside the calipers?
something else? I'm at a loss.

P.S. I'm 99% sure it's the brakes and not the suspension; even on the
tiniest 2% braking I can feel the thunk and repeat it over and over at
any speed, as long as the front wheel is turning. At a stop it goes
thunk once but doesn't repeat until I move the wheel.

DIFFERENT QUESTION:

Stock fork oil (I believe) is 10W-30. I put 10W fork oil in with a bit
of 15W (which was recommended by the list.... in fact most said use
straight 15W but I was happy with how the stock fork worked, and by
the time I had "half" of the 10W in--by my estimation--it was already
pretty much full). Anyway it feels much more active now, and bottoms
much more easily than I remember 4 months ago when I took it apart.

Am I right in thinking that 10W30 is 10W when cold and 30W when hot?
If so, wouldn't it make sense that the "actual" riding weight of the
fork oil when warm/hot would be somewhere between 10W and 30W? I liked
how the stock oil behaved... I'm not sure I like the new stuff. Any
input? Would crud/water/breakdown over the past 23K miles make the oil
feel thicker or thinner? Is that even possible to predict?

-- 
Utah Jeff
'96 SheePz1100