We were doing the same - running 87 octane every other tank or half tank premium/ half tank regular. However, after explaining to my wife that premium is only $4 more per fillup, she said it wasn't worth the effort and has been using premium ever since.

But to add to your comment, we never really saw a difference in performance between regular and premium.

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2001 X5 3.0i (PP/WP/AP) Traded at 213K miles
RARE White on Black - Why does everyone get Tan??
First E53 with 215s on XOutpost!!