I have a 2008 F250, 6.4l diesel with 140000 miles. Would you power flush or just an old fashion drain/fill transmission fluid???

 

A powerflush is impossible. I recommend this procedure: https://www.ford-trucks.com/forums/1...-exchange.html

Since there is no drain plug in the torque converter you can only drain the pan, which will only remove about half of the fluid.

 

Drop the pan and change the filter replace the oil. If you are worried about not getting all the old oil out drop the drain plug and change the oil in the pan every other oil change.
engine

 

I have a 2008 f250 6.4. I let the Ford dealer do the hot flush every 90k. They said the filter is lifetime. I'm at 220k right now and never had any transmission issues.

 

The pan filter definitely not lifetime.

 

There was a running change in 2008 that changed the pan filter from lifetime to needing a change. If yours was built in the first half of the model year it was lifetime. If it was built in the second half you are way overdue for a new filter.

 

I did a spreadsheet once....it takes 17 drain and fills to get 100% of the old fluid out.

 

The method Mark K references works really well, and is pretty easy to do.