BRAT 85 1.8 engine. After retrieving from three months storage the oil pressure cage stays at zero. Prior to storage it would go up to 30 when cold and drop to 15 when warm.. 

with the sender disconnected gauge reads 0 lead shorted to ground gauge goes to max. Ohm meter on sending unit shows infinite with engine off. Engine on ohms stays at infinite.

With engine on I would expect some resistance to be indicated, I think. Do I have bad  sending unit.Taking to shop to have mechanical check of pressure at new pump.

Could be gummed up inside? try rubbing alcohol and shaking it up...

Sounds like the sender is no good.  The sender for the gauge should  always have some resistance I would think.  The fsm probably has a detailed ohm check.  I'll have to look it up.

It's a pretty odd thing.  The sensor is a bi metal thing with a contact.  The other contact is on the diaphragm that is moved by oil pressure.  The meter is a bi metal arm with a heater.   The sensor heater is in series with the gauge heater.  So when the sensor heater bends enough to open it's contact, the gauge bi metal bends a corresponding amount, moving the needle proportionally.

It is normal to be open when at zero pressure.  The FSM also warns NOT to apply 12V to either heater directly, as excessive current will flow and could damage them.