re: "...The 240 I own requires minimum 90 octane...."

Most of the later n/a RWD and FWD (at least up to '07) that I have experience with called for AKI 91 (essentially a new name for the same USA's octane rating), at least to avoid retarded timing in response to knock and which lowers fuel economy (and the "fun factor" of power output, little to spare on a RWD car).
[sidebar: speaking of which, there were a few reports in this forum a while back claiming that, at least as applied to hill country where you need that extra oomph, the price of the higher octane can offset the cost of the reduced mileage to almost break even. However, we can assume that if you drive gently and live in a flat area, you can make a profit by using 89 (despite being below optimum AKI) because the anti-knock protection won't be called upon often enough to appreciable hurt your fuel economy.]

I face a similar situation as you: in my part of the country, you can buy either 89 or 93.

Personally, I just put in straight '93, because I drive hard and want as much of that "fun factor" that I can wring out of a n/a red-block. But you can blend your fuel if you wish -- it will work.