You shouldn't need to flush the system to either use a full or blended synthetic oil. The issue with the flat tappets and older engines is a little more grey. Some say that these engines need zinc in their oils to reduce wear. Some oil company reps say that their oils meet all of the requirements of these engines as well as all more modern stuff. In any case there are some oils, mostly in the 20w50 viscosity range that still contain ZDDP and there are also ZDDP additives that can be purchased to add to your oil.

For more information on this subject do a search on ZDDP and read the comments. A lot of good information but as usual you got to wade thru it to get to it.