I believe every 3k miles. I do mine every time I change the oil in the truck.

 

Andy: don't you have a higher-stall converter? That's the problem if you do. IIRC we have the same size tires and 3.73 gears and I average your hwy mileage on my commute (mixed 40-60mph with several lights and stop signs on the way, with a bunch of awful drivers)

 

I will get 13-15mpg average per tank depending on how I drive it

 

I'm not Procharged for those who don't know, but Andy isn't yet either. I'm just here to learn

 

I DD mine. D1sc 12lbs. Stock 3.42s and a 3200 stall. FLT lvl7 4l60e. Drives good maybe 20mpg on the hwy.