89 should be fine.  I believe the compression on these engines wasn't really high like the early 350s, so you should run fine on mid-grade.

the best fuel to use is PREMIUM fuel--93 octane--for the best performance and fuel mileage.  You can use 89 but it will not perform to the best of its ability no matter what anyone else thinks or says.  I have performed quite a bit of in-depth comparisons and now only use premium in mine.  The cost difference is not worth the difference in performance.

87 is ok for daily driving. That is all I use.

How about using regular grade gasoline and adding a octane booster product for each tank? I did this with my 2000 740iL BMW and the pings went away with a during acceleration.
dmajor