So, I was curious about trying a little high octane gas in the car, after reading the timing maps for the 370 and thinking that it might allow for a bit better timing advance by the ECU (up to max programmed potential). I know some other world regions have higher than 93 octane recommended and I know that the 10% ethanol lowers the effective octane rating a bit from 93 to probably 91ish.
Long story short, I made the hike out to a Sunoco about 35 miles away and put in 6 gallons of Sunoco 260 GT 100 octane gas. Mixed with the rest of the tank filled at 93, its about a 95-95.5 octane total in the car now. The Sunoco 260 GT burns very close to the stoichiometric ratio that pump gas with 10% ethanol does (14.0 vs 14.9ish) so its a safe option. I read several reviews online of this specific gas being completely street legal, safe for street cars and mixable with pump gas without issues before I tried it. It is fully unleaded, has no metal compounds to mess up sensors/cats, etc. Downside is that it is $8.29 a gallon!
Driving impressions: To get a before and after, I drove the car nice and hard before adding any of the 100 octane. I also tested it out under part throttle to look for responsiveness/etc, heavy throttle, etc. All seemed like it always did.
Once I filled up with 6 gallons of 100 octane and the rest at 93, I drove for a bit and did honestly not notice any change for about 10-15 miles. I also did not see any difference under light throttle/etc at all.
HOWEVER, after about 20 minutes of driving, I went ahead and got on it a bit more. The car pulled a LOT more responsively under quick throttle jabs, like it had 10-12 lbs more torque in the lower rpm band. I figured it might be in my head so I found a nice clear area and let her loose. I was not wrong, the car reeled back through 1st, lost traction completely into 2nd with a bit of a slip to the right, 2nd pulled just as hard and I spun again a bit into 3rd. The car absolutely did NOT do this before the high octane gas was in. I was thoroughly impressed. Driving around normally again was not much different, although the car seemed to respond to throttle input right away now, instead of kinda chumming a bit before going as it did before when it was hot. Oil temps were at 210 as well when I got it on and was testing it (before and after the octane switch). I did another WOT run with the same results as the 1st, pulls harder and the throttle is immediate.
I'm obviously not going to keep the gas in there all the time, but I'll be putting a few gallons in every now and then because the car seems to really like the 95 octane-ish blend. I doubt it needs more than that if anything, perhaps it only needs a "true" 93 octane with no ethanol to run at its best. Either way, I think the blend I have in works about as good as it can.
DISCLAIMER: I dont care if you think it was a waste, I know my car and I wouldnt bs myself and spend $8.29 a gallon for nothing, I was actually expecting to be disappointed. Negative comments or "its all in your head" will be ignored.
If you dont agree, stick with your normal octane.