White-faced gauges? Surprised no one has asked this yet. I had an overlay that said "Premium fuel only", and ended up swapping the whole instrument cluster with one from an 01 Cobra.

Is the instrument cluster factory? (Black-faced, red-line of 5750 RPM, 150 MPH speedometer) If not, it is probably from a Cobra. See pictures below.

GTs take 87 octane fuel. If you put 87 in and it didn't ping or detonate, nothing to worry about. Just be mindful of it for a while. If it goes without problems, then it takes 87.

My old, factory instrument cluster
[IMG]local://upfiles/20167/AC419C07044E4F1BA71EFBB0BD4C0F95.jpg[/IMG]

Cobra instrument cluster, in my car
[IMG]local://upfiles/20167/989CC276A4C44ED8A84C5CE328E0D286.jpg[/IMG]

Edit: Sorry about the blurriness of the second picture, was just with my camera phone, I just had to take a picture of the odometer at 0.5 miles It says "Premium Unleaded Fuel Only" below the speedometer.

Edit again: After re-reading your original post, you read in the manual that said it required 87 octane, so it takes 87. Either there is an overlay on the instrument cluster or the instrument cluster was replaced at some point (and the mileage may not be correct).

 

Those are the exact gauge faces on mine and my wifes cars(hers is the white faced that has the premium writin on it.

 

It appears to be factory also.

 

Nevermind.

 

The manual in my 04 stang says 87 octane and nothing on the dash my wifes 04 stang is the one that says it.

 

It isn't factory (the guages in your wifey's car).

Run 87!

 

Thanks