I'm an environmental engineer and geologist with some background in chemistry and have been cleaning up the mess from gasoline and other fuels and organic and inorganic compounds like benzene, toluene, xylene, MTBE and lead from sites all over the place for about 20 years. We made a big mess in the past, there have been and still are lots of problems and how we use and manage fuels does make a difference and there is good reason to try to get it right.

Insanamine's comments on octane are consistent with everything I've ever read. I would add that leaded gas also allowed for softer valve seals cause since you can't get leadded gas anymore, I have to add a lead replacement when I fill up to my '66 to avoid burning the seals. I've always assumed the lead was also a lubricant and that the softer seals require it or its replacement.

I've tried Exxon and Mobil 91 and Sunoco Ultra 94 in my Z. I started with Mobil 91 for 2 or 3 tanks full. I ran the last tank down and the first time I used the 94 you could feel the ECU adjust to the difference after about 10 minutes and it resulted in a positive change in performance, smoother revs and better acceleration when you punched it. After a few months of 94, I tried the Exxon 91 and then the Mobil 91 and noticed, I think, a slight drop in performance. I'm back on the 94 now and probably will stick with it - maybe its placebo affect, but that's my butt dyno opinion.

I'm wondering about all the other additives gas companies put in gas. You don't see much about them at the gas station but I'm wondering if they don't make more of a difference in performance than a few octane points will.

 

Hmmmm Maybe you're right, but its moot now. I sold the car and the Z and the Maxima both like 91, Nissan says so. I find that Nissan is always right, don't you? I gave it up when they added Paraquat.