2005 Ford Taurus

Engine light came on, diagnosed as multiple misfire. What could I do about that?

Find the missfire and fix it.

If you “stepped down” the octane of the gasoline you use, “step it up” to the recommended octane.
Have you recently changed the spark plugs? If not, now would be a good time.

I had a similar problem on a 1996 Windstar with the 3.8L engine. The exhaust gas recirculation ports were clogged with carbon. The dealer cleaned them but the problem recurred. I used a gasoline additive to clean the EGR ports and that worked for a while, but it did not resolve why the ports became clogged so quickly. You might try an additive to see if it helps. The one I used most frequently directed you to use two bottles in an empty tank, then fill up for the first treatment. Then use one bottle at each fill-up if you need it.