I would be very suss of the alternator, mine did the same thing, voltage on gauge got lower and lower, than dropped off, light came on, alt had s**t itself. Even though it now sits on 13 , and up to 14, it still charges fine, according to my multi meter.

Yeah i notice this all the time with my XR6, this morning was nice and cold down here so oil pressure was around 3/4 half way then as soon as it warmed up in the arvo it sat straight on the 2nd to last bar on the gauge.

I have about 3000kms till the car needs a service so its still going alrite, oil hasnt changed colour so thats a plus i guess.

i use penrite semi synthetic, sits between top 2nd and 1st bar whilst driving, drops to bout half on idle after warm.
do know if the connection for the oil pressure sender doesnt have good contact it reads very high!!!
id try another oil, im not sure what oil you use now but possibly a better brand.

Im also using the Penrite HPR 15 semi synthetic oil And i have also noticed the low volts when the oil pressure guage is low too. Mabe it is something to do with the alternator

My car ticks a little bit and it is audible sometimes but it runs fine and the oil is a golden colour and is still fairly think.

Very interesting. I had exactly the same problem as you guys before. But as soon as Tickford_6 suggested I use a thicker oil, I immidiately switched to Penrite HPR15 I think that was 15-60w thick. Didn't have a problem with oil pressure going very low after that. But yeah, I still noticed that whenever I turn off my airconditioning, my oil pressure and voltmeter reading goes up a knotch.

at 150,000k's you should definately not be having trouble with worn pumps etc. I would advise you to see a mechanic immediatly.
i'm having terible trouble with oil pressure now with an engine done 230,000. but i suspect thats because the previous 'abuser' didn't know the meaning of service!!! :
if it's worn out bottom end components in the engine, i would recommend wynns 'charge' oil additive designed to help keep oil viscosity at higher temperatures. i have put this in mine to attempt to keep the pressure higher. and i have noticed better guage readings since then, yet it still sits just below the 2nd bar at 2000rpm, plus the lifters clack and occaisionally it makes a wonderful scraping sound from the front of the motor (suspected as not enough pressure to tension timing chain).

I drive an EF II fairmont.
I have a few ideas about this and a possible solution.

First of all I had this really really bad last Summer. Driving around in 40 degree temperature. And at red lights, the oil guage would go really low. Made me hot just looking at it, and I could then hear every noise the engine was making, and it did seem rough.
I always noticed that the volt guage was low at the time this thing was happening.
So I got some opinions, and decided to look for earth leaks and bad earths amongst other things.

Heres what I found and you can do.
Remove air box lid, intake tubing and air filter. Now undo the bolts and remove the entire airbox housing. Just behind the passenger side headlight, bolted to the body is one of the few earth points around the engine. This one can get seriosly oxidised and green. Undo this earth bolt and give the bolt and the car body it's attached a good scrub, so it gets good contact again.
This move improved my low oil and volts symptoms like you wouldn't believe.
Unless you have other problems with the oil guage, this just may work for you.

It makes sense that if the cluster has a power drain caused by a load on the electrical system. It's going to affect some of the readings in the guages.
As you can tell, this all happens on 40 degree days. When you happen to have the air-con on, and the battery is very hot and is hardly being cooled by the restricted air flow of out cars. You probably have the stereo blarring away, if it's at night you'll have the lights on if not high beams. It's a huge load on the cars electrical system. And I think this is why it happens.

Hope this helps someone,

cheers.

Yeah, it does make a lot of sense. I checked mine first thing today, but the bolt was ok, tight and clean. I still gave it a clean though just to make sure. I got a bit surprised though by its green color, but found that my other secondary earth points-bolts are painted green. Hmmmm interesting. Anyways, in this light does anyone know how many earthpoints we have in the engine bay? Are there anyother earthpoints outside the engine bay...in the cabin perhaps? or am I starting to ask stupid questions now.