Skip the flush. All it will do is thin the oil and remove the sludge quicker. The problems associated with synth. and sludge removal are gasket and seal leaks on higher mileage or sludged up engines. All a flush will do is accellerate this, if it's going to happen at all. Considering the problems some people are reporting regarding bearing failures I can't recommend putting a thinning agent in the oil. Just go synth. and then if you develope leaks you might want to try one of the conventional oils for hi-mileage engines that rejuvinate (read SWELL) the seals.

Old school mechanics used to add some trans fluid ( very high in detergent) into the oil fill to help clean the internals, but again, it will also thin the oil.

Another option: If you want you can go synth. and then just do your 2nd synth. oil change early, like 500- 1000 miles.