Started my '71 240z this morning for a drive.  While idling noticed the oil pressure gauge was sitting on zero.  Shut it down, checked oil level (good).  Started again and removed the oil filler cap, looks like lots of oil being slung around.  Maybe the sender has failed, maybe the wiring, maybe the gauge.  What's the next troubleshooting step?  

Remove the wire from the oil pressure sender and short the wire to the chassis. The best way to do this is have a wire with an alligator clip on one end and a normal male bullet connector on the other end. Clip the wire to the shock tower or negative terminal of the battery and connect the bullet to the wire for the oil pressure sender.

Go in your car. Put the key in ON (don't start the car) and watch the oil pressure gauge. If it shoots up, you probably have a bad sender.

Definitive diagnosis would be to confirm with a mechanical gauge. See this thread for info on installing a mechanical gauge: 

 

So the Glow Shift adapter, which goes between the block and the sender, arrived today.  Stupid Q: do I need to drain the engine oil before removing the existing sender, or is the sender above the oil level in the block?

no need to drain the oil. sender is above the level. 

7 hours ago, kenward1000 said:

So the Glow Shift adapter, which goes between the block and the sender, arrived today.  Stupid Q: do I need to drain the engine oil before removing the existing sender, or is the sender above the oil level in the block?

Also, don't use teflon tape on the threads. If you do, you'll likely insulate the stock sender from the block.

11 hours ago, kenward1000 said:

So the Glow Shift adapter, which goes between the block and the sender, arrived today.  Stupid Q: do I need to drain the engine oil before removing the existing sender, or is the sender above the oil level in the block?

As mentioned, the oil pressure port is above the resting oil level in the crankcase. Note where the dipstick is, and the oil level on it when checked, in relation to the location of the oil pressure port.

Just don’t try to do it with the engine running.

 

 

You will see some leakage from the oil galley but it should be limited