You can do without a flush by doing what I did. It should do the same...

Do a regular oil change. Then drive about a 1000 miles and then do a synthetic oil change. I did it this way and haven't had any problems with it.

Regards,
Mike H.
--
1998 Nautic Blue C70 - minor mods here and there...and HID!!!

I recommend use of a good engine flush before changing to synthetic in any car with over 20,000 miles or so.
The professional flush machines do an excellent job, but doing it yourself with a good flush such as made by AMSOIL or BG works fine (and is a lot less expensive).
If you don't, or won't, flush, the synthetic will clean the engine for you, just more slowly. In that case, you might have to change the oil filter after a couple of thousand miles.

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.

How many miles on the car and what styles of driving and what brand/type of oil was used and what were the oil change intervals?

Flushing cleans some of the crap/scum/oxidized coating left behind by petroleum oil and by the normal combustion process. Not 100% required and synthetic oil will start to clean this stuff, but as it's cleaned, some of the crud will be combusted along with some of your oil....in a particularly dirty engine this oil consumption can be high for awhile then it drops off.

Flushing your crankcase will help get through this consumption period faster...by "precleaning" the engine for change over.

For high miler engines read this (I know this may be way overboard for a "newer" 850, but you can take out of it what you want):

http://www.fidalgo.net/~brook4/ENGINE_FLUSH.html