SORRY LAST SENTENCE SHOULD READ;

Could this be due to refineries now using heavier oils and/or insufficient supplies of lighter crudes like Brent and/or WTI?

This could say something about present availability of sweet crudes.

US oil production according to TWIP is down approx. 400 kb/d relative 2005.

It's primarily because some refineries are still down from the hurricane. We have never gotten back to full capacity. See this graph for refinery utilization, and you can see that we haven't recovered:

http://www.petrostrategies.org/Graphs/refineryutilization.html

RR

Sulfur content is about the same as 6 years ago.

The same with viscosity

Clearly since 1985 there has been a deterioration in both areas - how significant this is, is unclear.

Thanks for the link to data on refinery inputs. What I think that this illustrates is that there was been limited [if any] improvement in the ability of U.S. refineries to handle higher amounts of heavy / sour crudes.

If production is shifting toward these heavier sour grades, "Houston we have a problem" [an appropriate city from a quote coming completely out of context. Depending on your degree of faith in the U.S. Government feel free to substitute "Washington" for "Houston".] :-)