As soon as I hit 1000 miles I will change it over to Mobil 1 Synthetic. Then change every 5K miles. In all of my vehicles i usually changed engine oil every 3K and reflushed transmission fluid every 20K. With synthetic we changed the interval to 5K on oil changes. My father taught me this and I bought his 2001 Grand Marquis at 201K miles. It started to nickel & dime him and Mom wanted something newer. Found a 2004 Grand Marquis 1 owner and serviced regularly at his local dealership. Carfax report was very good and he ended up getting it for $4200. I gave him $1500 for the 2001, drove it back to Tennessee and gave it brand new Bosch spark injectors. That was 2 years ago and i only put 11K miles on it. Point is Transmission still shifts smooth, engine runs well, and for long trips (since its a hwy cruiser) i get 29 mpg.
Hopefully I can have the transmission fluid changed every 20K as well on my new Ram. I'll find out at at 1K mark since i want to change the transmission fluid from the factory as well.