If your car has an EGR controller, you may be able to tap into the RPM signal to lockout the oil pressure alarm. I say this because my 83 240D has an EGR controller which has circuitry to measures RPM to control the EGR valve and I have tapped into it to drive a tach.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked