I replaced my oil pressure sender on my 77 280 and it now seems to read backwards. Is this the aftermarket sender.

I replaced it with another, also aftermarket, with the same result. Is this a problem with aftermarket senders?

What brand(s) are you buying?

from NAPA auto parts. Echlin

It reads max oil pressure when the ignition is turned on and goes to less than zero when the engine is started.

Well, you could try to see if the sender is still available from Nissan. I can't remember whether or not it is. I haven't purchased one in a long time. Also, the FSM says to use a conductive sealer. The first option that comes to my mind is antiseize, but give others a chance to correct me before you try using that.