I know I'm going to be crucified but here's my answer.

When gas prices shot to hell and back I started mixing.

My theory, I'm probably wrong but it's working and I haven't noticed any differences or problems.... anyway

The manual says 91 octane, here in TN we have 93. OK, so when prices went up here's what we started... I'd fill up with 93 then when it got to half a tank I'd top off with middle grade/89 octane.

Let's do the math.... 93 + 89 = 182 now divide that by 2 to get the average and you get ... 91 !!! Just what the manual calls for.

Now when it gets to half a tank again it's back to 93. Back and forth unless I let if fall too far below half a tank then full 93.