overdrive-wrong fluid?

Hi...after transmission heats up (20 minutes) van will not go into overdrive. This started happening right after had transmission serviced (flush and filter). Van does not throw a code, OD light does not blink, check engine light is not on. My husband thiks the wrong fluid was used, and when the fluid gets warm it screws up the overdrive. the windstar needs a specific transmission fluid. The place that did flush says it used correct fluid my husband is starting to doubt that. Any ideas?:banghead:

Late model WS vans need "Mercon V" ATF, most shops (even some dealers) cannot get this straight.

This applies to the '98 and newer 4 speed (3 + OD) equipped vehicles.

Have you checked the level?