Es müsste "Doorways - Die 4. Dimension" (1993) sein.

Laut einem englisch-sprachigen Kommentar aus dem IMDb.

Ich poste es einfach mal hier rein, von RE ZULETA (06.03.2006):

"I also remembered, in one segment of this 2-hr cable movie...the story was sorta similar to the world of 'Kevin Costner's THE POSTMAN;' in this version, in order to cure some oil-spill catastrophe in the world's oceans...scientists had created this petrol-cured oil eating bacteria. But when they unleashed it...things got outta control, and it consumed all the oil-supplies in the world. So, now supposedly everything went back to being kinda the old west. Traveling in horse pulled buggy-coaches. Horse back riding. Seeing abandoned luxury cars on the side of cracked freeways."