Octane just measures a fuel's ability to resist predetonation. That said, fuel efficiency aside, the proper octane for the M103 engine (or any other engine for that matter) is the lowest octane you can run without experiencing pinging. If your particular engine doesn't ping using 87, then 87 is fine. If it pings for whatever reason, just use the next higher-octane fuel until the pinging stops. If 89 gets the pinging to stop, 89 is fine. But you may require 91 or 93. The "premium" fuel requirement in the owner's manual is put there as a general rule for owner's who don't know the difference.

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280