My 77 280z seems to be running high on oil pressure for some reason.

The pressure was running 12-15psi at idle when warmed up and around 55-60psi when driving hard and around 40-45psi when cruising.

Now it's at around 25-30psi when at idle and seems to climb to almost around 75psi when driving.

The motor still runs as smooth as it was before the high oil pressure readings both at idle and in gear but now has a slight but distinct clanking sound that wasn't there before. I can't seem to figure where it's coming either from but it's best heard if I listen underneath the car.

Only thing I was messing around with prior was the AFM but I wasn't even making adjustments, I was just fingering it to enrich or lean out the fuel/air mixture to see if the motor ran any differently, followed the directions on the Atlantic Z site. I'm getting really bad gas mileage. Anyways this wouldn't mess with the oil pressure right or could it?

Other than that, haven't done any mods to the car except upgrade to a 280zx alternator. But the oil pressure was fine after the conversion.

I know the oil pressure sending units go bad but don't bad sending units usually read low rather than high?

But then again there is that noise....

Any suggestions on where I should begin diagnosing?

What wieght oil are you using ? And you said you are using an autometer electric oil pressure gauge are you using the stock sending unit or the autometer unit ? And I don't think changing your oil filter will make any difference if you have not bypassed the oil filter relief valve.

Castrol GTX 30 weight and I've been using the same stock one since I bought the car.

Ok, if you are using the autometer then you need to get the proper sending unit for that gauge and try 10/40 weight and see what happens.

John, the weather has been slightly cooler this month but still relatively warm.

FastWoman, the pressure does hold steady once I get around 3500rpm but it's around 70psi and around 25psi at idle. But after I drive the car around for 20 minutes or so the pressure drops back to normal, which is 55ish psi driving and 15ish psi at idle.

There is nothing wrong with the oil pressure readings you are seeing. Its absolutely normal and is nothing to worry about. You don't need to do anything at all except stop worrying.

What John and Zforce said! Your pressure relief valve is doing its job. I'll only say that 15'ish is a bit low for a warm idle. It might be a sender/gauge incompatibility. It might also be that your oil is too thin. Both my current Z and the Z before it did great with 10W40. When I lived in Texas, I would fill with 10W40 and top off with 20W50. That kept the low RPM pressure up towards the end of each oil change cycle.

Horsemann, I did the oil change not too long ago, maybe 1000 miles ago and I have always stayed on schedule with the changes ever since I've had the car, which has been about 5 years.

I think I will change the oil out again soon though and run some of the gunk flush before I drain it. x

Just because it is new doesn't mean it is good. if the filter isn't flowing freely, the pressure rises enough to bypass it, hense the SLIGHT rise in pressure you are seeing. I would try a filter replacement before you go pouring sludge buster in the motor and causing new and unecessary problems. FWIW, I think I would just do as John C. says, don't worry, be happy.