Unfortunately, I actually tested my VDO dual sender on the engine I blew
up.  The red light came on at about 3 psi, the same as the switch did.
Perhaps they make different versions of senders, I can't explain the
discrepancy.  You can rig up some fittings and tubing to hook up a
manual gauge where the sender connects if you want to test it.

I really doubt that your gauge is wrong, and your readings are exactly
what I had on my engine before I threw a rod.  20 psi at speed is not
good, there should be a minimum of 10 psi per 1000 rpm, or 30 psi at
speed, 9-10 at idle.  When mine dropped to 15-18 at speed the rod went
within a few hundred miles and destroyed the engine.

Rebuild the engine while you still have one to rebuild!  I paid the
ultimate price for the denial you now have.

Stuart