If you have a tune for 91 you can put 93 in with no issues. You would only be saving a few cents trying to blend your own, it wouldn't be worth it.
'14 Tuxedo Black ST3
'13 Ingot Silver Escape Titanium AWD
'04 CD Silver SVT, #877, 1 of 135 May 2011 ROTM, FF11: Best Display, Best of Show; AFN12: Best SVT, Best Audio, Finest Five, Carlisle Nifty Fifty