I believe that the 5.6L V8 doesn't have adaptive knock control, so you should hear via pinging if your octane is too low.

As your engine ages and accumulates some deposits (all do), your octane requirement may creep up. My 88 BMW 735i was spec'd at 87 octane, but I found after 170k miles that it took 89 octane to keep it from pinging. The pinging was only noticeable at high RPMs and heavy throttle - and I backed off as soon as I heard it.

Higher compression engines require higher octane - so the 400E/E420 engine, with an 11:1 compression ratio would require a higher octane that an engine witha 9:1 compresison ratio. Check and see what the 5.6L ratio is - it's probably 9:1, and 87 should be OK for it. Run 89 just to be sure.

Troy
1995 E420 75k