I agree with dk.Get a correct reading with a manual gauge.
Also,I have a 03,I have replaced my sending unit awhile back.
However,the gauge itself sticks at times. I tap it on the side and
it gives a correct reading. I've talked to other Z owners of 03-04
cars and some have reported similar problems with the gauge.
How many miles on your car? THe sticking gauge problem on my car started around
75-80k miles. It now has 96k miles.
Uses zero oil between changes!