I'd still be inclined to flush first. Even the cheaper blends have a tendency to have a bit more detergent in them .
If nothing else, use the K&N motor flush, or the Gunk flush ( not as good)it usually can be found at Auto zone , Pep boys ,etc.
If you are worried about a flush harming seals, right after flushing, put on a cheap filter and fill with cheap wal-mart oil and drive it around for 1/2 hour or so, then drain it and change filter to the good stuff.
This will get the remaining flush residue out