Ben,

I think the .9 bar warning, at speed, is just that, a real warning of
possible catastrophic engine failure.

The 2.0 bar minimum is the minimum operating spec for a normal engine.
Anything below this, and you still have low oil pressure, a problem for sure
but maybe not critical.

Now, you may still be having a problem with the high pressure switch, if it
sets off at 1.4 bar or the similar. Still I think there is a problem with
the engine as well- 20 psi at speed doesn't sound good to me. I'd agree with
the Bentley, and want to see something like 30 psi.

Good luck

Tom F.