If you run 87 the engine will be forced to use the knock sensors to constantly de tune the engine. I don't recommend regularly relying on decades old hardware to perform that function like new. Also I can't tolerate the performance hit. I will mix 93 and 87 to get 90 however

Oldskewel, great pic, great idea the switch. But is the longer plug in the foreground of pic at the top, the Bank 1 sensor post cat? I can't recall the precat O2's it's been so long. Thanks.

Those connectors there in the pic are for both banks, both post-cat. I don't remember which is bank 1 vs 2. But as you'll notice, due to their bends and locations, you won't need to worry about mixing them up. If it matters, you could probably disconnect one of them and see what codes appear.

The pre-cat sensor connectors for both banks can be seen here, along the starboard side of the head.

Attachment 76966

Also, here's a better pic of what the post-cat connectors look like when you first see them, clipped in place, cables routed, etc.:
Attachment 76968

Oh, that's right. Two top, two bottom on the head for precats. Thanks for the reminder, I guess I should have looked more closely when checking the oil the other day.I removed the silly plastic shield years ago, just jams up head heat from escaping. Miss the old E30 days of solid aluminum v.covers.

Miss the old E30 days for A LOT of things. And they all cost a lot less money/time to repair than the E53. :bustingup