I noticed the same thing (brown inside the filler) when we got our 854T at around 198,000 miles (now at 232,000). The PO did have regular oil changes.

I ran 3/4 - 1 quart of ATF in it for 500 miles 2 or 3 times to hopefully clean it up. At the time, the car was being used 500 miles per week on the highway so it had a good chance to work.

There has not been any negative effects from this treatment as far as I can see. It cleaned up a little, but certainly doesn't sparkle inside. It may have helped the PCV system some, but that's too hard to tell. The car was a pretty good oil burner when we got it and consumption / burning didn't change. It's still a 700 mile per quart car.

I hope to do the valve seals by the fall.

Edit: Come to think of it, I did the same treatment on a B20 years ago. Under the valve cover was awful and though still dirty, there was a remarkable improvement.
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"Differences of opinions should be tolerated, but not if they're too different' - Sharon Craig