I traded in my 06 G35 two years ago with 171796 miles. It ran like new when I traded it in.
Almost all of those miles were were on 87 octane.
I did a one month test of 91 octane and a one month test of 93 octane in April and May 2009.
I did not notice any increase in performance but did notice a slight reduction in MPG
I am almost at 47k on my current 2012 G37S so having driven an Infiniti for close to 219k miles and my current 2012 G37S has never seen a drop of anything other than 87 octane I can tell you that you need not worry what grade of gasoline a previous owner used.
There is no need to waste money on fuel treatment.
I am also on the road in the early morning hours earlier than you and I track my MPG very carefully with detailed records over almost nine years.
I calculate my MPG by miles traveled/ gallons used to fill up.
Here are the vehicle computer results of my round trip from NJ to Florida in June
You can see the results of the four fill ups on the way down, the not so great MPG wile the wife drove on many short trips while in Florida and the four fill ups on the return to NJ. All on 87 octane filling up at large truck stops such as Petro,TA,Raceway, and Hess. I did use Shell on one fill up because they had the cheapest price among several stations.
The last fill up on the return in Ashland, Va to NJ gave me my best MPG of the trip at 28.49 MPG
Stop worrying about what grade of gasoline to use and enjoy your G.
If you enjoy pissing money away use premium.
If you are cheap like me use regular 87. It will not hurt your G
Cheers
Telcoman