Unless the durango is being used for racing or towing in the desert at high speeds, u don't need a syn. Todays oils, rotella, havoline, etc are all basically semi-syns and are good for 5-9k mile oil change interval if being used in non-severe conditions. The longer oil change of amsoil has always been based on changing the oil filter, replacing the lost oil or using a by-pass filtration system. Syn oil gets just as dirty as non-syn oil.
By-pass filtration is the key to long engine and oil life even with non-full syn oils (group iv and v) . Actually all full syn oils are a combination of both groups.
suggest one look into the amsoil bypass oil filtration system or the toilet paper bypass filtration systems.
www.bobistheoilguy.com has many threads on this. Changing the oil every 2-3k miles does the same thing, which is removing the contaminents from the engine that cause wear.
I use pureone filters and mainly group III oils, with always 1 qt of amsoil, for the cleaning effect of the esters. also using
www.auto-rx.com
enjoy!