Oil Pressure issues
 

I have a 1987 N/A, recently rebuilt, and I'm finally getting into the break in stage. Initially the oil pressure gauge wasn't working had to do some work with the gauges but eventually got it working. Now that I'm driving it around finally I've noticed that when the car starts, oil pressure jumps straight up to 60, seemingly normal. As I drive it however, get into 3rd and 4th oil pressure drops off, to 30 and then to not reading at all. I have cleaned the connection and replaced the wire on the sending unit with no affect. the car doesn't seem to be being affected so I assume it's a false reading. I plan to replace the sending unit but just wanted some verification before hand, just in case. Thanks in advance

You need to utilize a much better gauge than the stock Mazda cluster provides.
There are two options I can think of...

-temporarily fit a mechanical gauge and run around for a bit and see what it says. This will give you an idea of how accurate the stock gauge is.

- install an electric gauge and bypass the stocker altogether. This would be my preferred approach and I'd also replace the water temp as well. I understand the S4 temp gauge is supposedly better than the S5 but wouldn't trust either for such a critical function.

I've already replaced the temp gauge with an aftermarket one and it's working much better, thanks for the advice I'll definitely look into it. I can imagine after 31 years the sender unit is probably on its last legs, as well as the gauge. I doubt it's ever been replaced before (the previous owner had no idea what he was doing, had all trailing spark plugs installed if that's any indication) the car has a s5 back plate which is why I got a new temp gauge as the thread pitch and connectors were different for the sensor I don't know if that could be affecting oil pressure.