I know this subject has been touched on before but I thought I'd share my own experience. My NF Fairlane would show low oil pressure during hot weather and sound the alarm. The car has 420,000 kms on it so I thought I'd try some thicker oil. This did nothing to improve the situation. I remember reading that it could be an earthing problem in the engine bay. Popped the bonnet and started the car. Turned the headlights on, highbeam on and A/C on to provide maximum load too the electrical system. Measured with a multimeter from the engine block to the negative battery terminal and was shocked to see 640mV or 0.640 Volts DC drop across the negative battery lead (engine block to negative battery terminal). This should never exceed 300mV with maximum load so I'd found the culprit. I fitted a newer lead from another car and tested it. This time only 84mV or 0.084 Volts DC. Thats more like it. Looking back another clue to this being electrical and not poor oil pressure was the fact that every time the A/C clutch engaged, the voltmeter on the dash would drop enough for me to notice it. It's funny how you peice everything together after you've solved the problem. I hope this stops someone else changing their oil when its not required.

I have exactly the same problem with the same car. Maybe thats what is up with mine too! Im going to check it out:)

Alfy, What car did you get the new earth strap from? Cheers:)

Got the replacement from an EF falcon at pick and payless. Did a good physical of the cars before I decided on the cleanest lead with the least corrosion and outer sheath damage. Of course with used parts there is no guarantee so I'm going to make a new one in the next couple of weeks and chuck the EF one in the bin.

Cool thanks mate, going to try to find a thicker one, Im lead to believe earlier models had a thicker better one. Cheers

when you guys tested, did you just touch the lead to any part of the block? the lead on mine has 2ohm or so resistance, but even when i scraped the block with the probe a little, it was giving unreadable resistance. i checked the connection to the block, corrosion, seemed fine?

I put one lead on the bolt that the earth lead connects to the engine ( alternator bracket rear bolt) and the other on the negative battery terminal.

I wouldn't use ohms to test this connection. A multimeters small voltage and current isn't enough to test this lead under load. As I said in my test, use your DC volts setting and turn on as many electrical loads as you can e.g. Head lights, High beam and aircon so you have maximum current flowing thru the earth lead. Now just measure the voltage drop between the engine block and the negative battery terminal with engine running and all turned on. If you get more than 300mV or 0.3Volts, then your negative lead is stuffed. Mine was only 0.084V or 84 millivolts when changed.

I know this is an old thread, but this was my exact problem.

Noticed that my oil pressure was low, even sounding the alarm when waiting at traffic lights in hot weather. So I switched to 20W-50 and that made a slight difference, but have put up with low pressure reading for over a year. Then I noticed that my voltage reading was also low. Thinking the two were related, I did a bit of Googling and found this thread.

Sure enough, I was dropping almost a full volt across the negative lead. Replaced it ($10 for a new lead from Auto One here in Broome), as well as both terminals as they were a bit old and dirty. Oil pressure is now well over the normal mark, which I assume is because of the heavier oil. The voltage reading is much higher, and hopefully should make my battery last longer.

So thanks heaps, appreciate your help.

Only thing is I noticed the lead to the starter motor was filthy with corrosion, even a few inches down where I cut it off to make a new end. Starter still works fine, but if anything starts to pack up with that then I'll replace it as well.

Sorry to drag up an old thread; but just wanted to post up a thank you.
A google search lead me here and fixed all the issues I had with oil pressure, voltage, and the air con.
Thanks muchly.