Without going into discussion too much about oils, what they can and can't do, I'll just give my opinion. Unless there is an internal engine oil leak or defective oil filter I would wager that your gauge fluctuations are an electrical issue to your gauge cluster. Basically the gauge or dash cluster is getting varying voltage. Well what would an opinion be without some way of verifying atleast some of it with a way of tests. The simplest and most straight forward way to see if there is indeed an oil pressure problem would be to Tee in a manual pressure gauge and either run it into the cabin of the car or tape it at the base of the windshield. The reason you want to Tee it in is so you can compare the manual gauge pressures to your dash gauge. For example if you run your engine at normal operating temperature with the manual gauge and your dash gauge sending unit hooked up. You get your hot engine oil psi, let's say its 35-40 psi while cruising down the roads (it will vary depending on engine RPM). Then you just compare your manual pressure gauge to the dash gauge. Say the manual gauge is 40 psi and your dash gauge reads right in the middle. Then you have to drive the vehicle until you notice your dash gauge go wonky. If the manual gauge pressure reads the same ie 40psi and your dash gauge is real low it's either a sending unit or dash electrical issue. If both manual gauge and dash gauge both start reading low then there is a problem somewhere in the oiling system. The cheapest and easiest thing to do is change the oil filter to see if the bypass valve inside the filter is faulty. If that doesn't work, some internal investigating may be necessary.

Last edited by MachTJ (10/22/2015 8:24 PM)

Oil is pressurised through a series of passage ways through out the engine block. When a clearance is too large or a passageway is not capped correctly they can leak. There are few possibilities for an internal leak to happen but they can and are pretty rare expecially an intermittent one. I wouldn't worry too much about internal oil leaks as a possibility.

I can't figure this out. My oil level is full. I checked the dip stick at idle and the oil level is way up on the stick. So pressure is good. Maybe the sending unit is screwy. I guess as long as the oil gauge is reading something. its all ok.
Thanks everyone.
James

wsinsle wrote:

I was thinking about this and thought of something you can try. All the gauges work the same, except the amp meter. You could flop wires between the temp and oil pressure gauge, The wires going to the sending units not the regulator, (CVR). If the symptom follows the gauge, you may have a gauge that is sticking. If not you elimate something.

Ok good idea. Thanks
Where is the cvr located?

Last edited by 13james13 (10/27/2015 9:21 AM)

josh-kebob wrote:

Attached on the backside of your instrument cluster....as two wires that plug into it.
The cvr is about a 1/2" wide by about an 1.5" long....

Alrighty, thanks Josh

13james13[b wrote:

]I read on here today's motor oils are not especially good for older, or older rebuilt motors.[/b] My original gauge, (new sending unit) in the morning sometimes slowly moves, other times somewhat quickly moves to half way but no lifters are tapping. Sometimes while driving the gauges reads a little over halfway, other times the gauge reads just a little under halfway while driving. Would today's motor oils affect oil pressures in these older motors? Thanks for your help.
James

Let's go back to the actual question you asked. Would today's motor oils affect oil pressures in these older motors? The answer is no.  As long as you are running the correct viscosity oil, you will not be able to tell current formulation oil from '60's vintage oil as it runs in your car.  Your first sentence probably should have been your question.  "I read on here today's motor oils are not especially good for older, or older rebuilt motors." Current formulation oils do not have the zinc antiwear additive that was used when the 60's Mustangs were new.  Without the zinc additive, flat camshaft lifters will wear at an accellerated rate and fail prematurely. If you are running flat lifters you should be running one of the specialty oils that have zinc in it, usually listed as ZDDP, or you can go to the autoparts store and buy a bottle of ZDDP additive to add every time you change oil.  Using the ZDDP additive allows you to use current formulation oil. 

Based on the information you have supplied so far, I doubt that you have a problem with oil pressure, the oil pressure gauge, or the voltage regulators. You just didn't understand what you were seeing.  That is alright, we all started with a knowlege base of zero. That is why this forum is here, to increase everyone's knowlege.

James,
MachTJ suggested testing your pressure with a manual guage.
Have you done that yet?  I'd do that first to confirm exactly what your oil pressure reading is.  If it tests normal, than you know your engine is sound so it's most likely a problem with that 50 year old wiring or dash gauage.
Harbor Freight is your friend, if you what to own your own guage set. For $26 it's yours; here's what I'm talking about or, check to see if autozone or another parts store has them to rent.  Until you check it this way, your gonna be chasing a fart in the wind.  Report back the results or if you need help running the test.

Last edited by josh-kebob (10/27/2015 2:54 PM)