问：启动时汽车口吃和低燃气里程

我有一个2013 G37，有45k英里，当我启动它时，它会稍微阻止，然后我想知道那是什么？Also my mileage per gallon has gone way down since I bought the car 2 months ago, it was about 26 per mile at first now it’s about 17 , I don’t drive crazy or anything Andy I always go to shell or chevron and get 91 gas .... also if I want to do a oil change myself what is the top of the line oil to put in the vehicle ? Thanks so much for the help

我的汽车有45906英里。
我的汽车有自动变速箱。