I would go with the Mobil 1 5w30, have no idea why the dealer would use 15w40 not the right oil for the 4.7 V8. If you want to know how much the 4.7 V8 likes the Mobil 1 5w30 check out the used oil analysis on other Toyota 4.7 V8's at
www.bobistheoilguy.com You will see that the Toyota V8 is very easy on oil.