I'm not sure if your pulling our leg or not, but this is a bad idea and no, it won't work. First of all, your sales person is wrong. The upper octane rating is 91. I suppose it may vary by region, but in my travels I have never come across a pump with 90 octane unleaded. Usually there are the standard three grades; 87-89-91 octane. Secondly, many of us use 89 or in the case of a 2010 ES, even 87 octane without engine knock or other problems.
Trying to mix two grades of gas to end up with a totally different grade is insane.
Paul
No, I don't think we're on the same page. Sales guy did say 90 is the most accurate grade for this car but I know 90 isn't available at any stations. Around here is 87-89-93. Orginal topic of this thread is about mixing 2 grades to make it 90 so all I was asking if I put in 50/50 of 87 & 93, will it mix properly and create that perfect 90.