May I add my experience to what Paul said below? No? Please stop reading now:)
I switched both of my cars to synthetic oil some time back. The Volvo (B230F) had 180K miles, and had had 2000 mile changes with Castrol. I cleaned the valve cover, as I had to replace the gasket prior to the flush and change, and found very little stuff crusted inside it. The stuff I drained out of the engine after the flush was pretty nasty looking. Afterward, the oil stayed pretty clean, and I didn't notice any increase in usage. My opinion is that you should use a flush.
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