It all boils down to personal preference, Yes the manual does state, every 5000 miles, but that also depends on your driving habits, and environment conditions..ie, if you drive you're vehicle on very dusty roads, or in very air polluted areas, then the manual recommends, that you change your oil every 3 months, or every 3000 miles, on the other hand, if those conditions don't apply to you, then it's recommended that you change your oil, every 5000 miles, or every 6 months..As for your dealer, being full of crap..No, not at all.. He's being completely honest with you...My dealer told me the very same thing, that I'm explaning to you, right now..You just need to apply good common sense and then, use your own judgement..As for myself personally, I'll be using Mobil 1 extended performance, which is supposed to be good for 15,000 Miles or once a year, which will be safe for me, being that I only put 4000 miles a year, on my Mustang..I only drive it during the spring, summer and early fall, during car cruise season and car shows..Other than that, My Mustang is garaged kept. in storage..