I have 4.7 V8 and like I would expect it will idle a bit high, probably around 1200-1300 when first stared but then keeps around 1000 when at idle in park. The problem I am having is that when I am in drive sitting at a light it will drop down to around 500-600 then back above 1000 and then stabilize this will happen a few times in a row or not at all not really depending on temperature either b/c it has happened at normal operating temperature as well as a bit cold. Also sometimes when I'm coming to a more abrupt stop is will do the same thing. I'm thinking (hoping) its only injectors or plugs and not something serious.
I doubt the dealer has equipment on hand to test the octane in your tank or the time to do it for that matter. Now e85 is another story is smells very different and is easy to identify. So don't make that mistake.