I have a dead S5 OMP, but several good working S6 OMP's. Is it possible to adapt them? I've heard they're electrically exactly the same with just different plugs. Obviously the oil manifold is a 4 port on the Series 5, but in theory two could be blocked off and just run two OMP lines on the Series 5 engine.

I'm considering giving it a shot unless someone has already confirmed it's not going to work. I could block it off... but OMP's are nice to have. In theory I could also block it off, and wire in the s6 OMP, and run premix as it would stop limp mode.

As an alternative approach - is it possible to repair the S5 OMP motor? Or potentially put an s6 motor on an s5 manifold?

 

so yes, basically.
like you say the difference is the connector on the harness, the number of discharge ports, and the FD has a little heat shield (FC has undrilled bosses)
the parts that are the same. Stepper motor, all the guts of the actual pump. the position sensor is also the same, but it is on the harness.

my car is running an FD engine, with an FC wiring harness, so i took the FD pump, and put the FC harness and position sensor on it.

if we take a step back there are three parts to the pump, the mechanical part, which seems to be really fool proof. the stepper motor, and the position sensor.
since you can swap the stepper motor, you might try that. the position sensor would be next easiest, but requires some harness work