I have a '79 F-100 inline 6. The oil has just been changed, and there is plenty of oil. But if I make a trip across town (15 miles or so), the oil pressure guage very slowly goes lower and lower and lower.
I haven’t been brave enough to see whether it will bottom out at some point, but is this normal for old trucks like this? One person recommended I use a 30wt oil instead of a 10-30wt. Is that a good idea? How can there be plenty of brand new oil but decreasing pressure?
Didn’t the pressure always drop as the engine warmed up?
How is the engine temp looking?
Also, at the age of the truck, the oil pressure unit/sender might not be accurate anymore.
If you’re really worried, get someone who has a guage to check it for you. Several years ago, I think on the early eighties thunderbirds ford had a tsb out for something similar. The customer would notice that his oil pressure read on the low side of normal. The customer would come in and complain. We would install resistor on oil sending unit to fix non-existent problem. Most newer fords have what I call a phony oil pressure guage. The guage reads right in the middle, unless you have no oil pressure. I guess they learned their lesson.
I’m not really worried; it’s just unsettling to watch the needle. The engine temp looks fine. I’ve had the truck about a month, and I think it’s something I just didn’t pay attention to before, but now that I’ve noticed it, I’m obsessed with it.
As engines warm up, oil pressure will drop. This is normal. If the oil pressure drops too far, then it is a problem. So the question becomes how far is too far? Good question, I can’t tell on your truck since I can see the guage from here.
Also as the engine wears, the oil pressure will drop. An engine with 100,000 miles will not have the same oil pressure as it did new. The wear on the bearings will cause a dip in oil pressure.
If you are seriously concerned, have a mechanical guage installed. This will/should read correctly. You might also take a look at the owner’s manual to see what it says about “normal.”
I don’t think 30W will show an improvement in oil pressure over 10W-30.
Have a look at the forums page at www.ford-trucks.com. I have found this to be a very useful resource for diagnosing problems like this.
What a great resource! Thank you! And thanks to all for the responses.
Since your location is indicated as southern US, you might want to consider bumping up to a higher viscosity oil such as 10W40 or 20W50.
I’ve had good results with increased weight oils in high mileage engines.
Having a minimum quantity of oil is necessary, but not sufficient, for having enough oil pressure. In other words, being low on oil is only one of many possible reasons for low pressure. And of course, as mentioned, having a low gauge reading does not necessarily mean having low pressure – gauge systems are not infallible.
My 1988 E150 does the same thing, sometimes. Sometimes it doesn´t. Usually it goes down to about 15-20 lbs and the check engine light comes on. (This normally happens at night on some back road fifty miles from nowhere, but I suppose that´s just because the gremlins know I´ll get upset)
A check with a mechanical indicator screwed into the pressure sender fitting shows 50-60 lbs.
I´ve changed the pressure gauge sender twice with no results so I suppose it´s just one of the many character flaws the truck is cursed with.
I´m noy buying any more Fords.
noy = not
Did someone mention “viscosity”?! 
My indomitable 76 Jeep CJ-7 has an aftermarket oil pressure gauge with a similar problem. I have my fingers crossed that the needle’s dive is due to a bad connection, a stuck switch or a faulty sending unit. With luck, the worst I’ll need to do is replace the gauge.
If not, it’s the cam bearings or the main bearings and I could be in for something like this:
Low oil pressure can cause a lot of damage to an engine. Until I know for sure, Chicory’s on light duty.
I don’t mean to be an alarmist, Liberal, but I’d hate to see anything bad happen to your new truck.
Hi Lib:
Congratulations on the truck! Hopefully you bought this truck out of a love for these vintage machines, and also a strong urge to tinker, rather than as a need for reliable day to day transportation.
What you have bought this truck for is really the key to determine how you should handle this.
What you have described is a very common symptom that may indicate one or more of several problems ranging from inconsequential to dire. That is why you are getting all kinds of different answers. You need to disregard anybody, include mechanics, who seems eager to diagnose this issue. If you follow that route you will end up replacing parts more or less at random until either the problem fixes itself, or you get frustrated and give up.
First you need to acquire great peace of mind. You may be dealing with a normal function of your vehicle, a bad gauge, a bad sender unit, the wrong viscosity oil, overheating, a vehicle displaying symptoms from a past severe overheating, a blown head gask, a bad oil pump, clogged oil lines, a failing engine (rings, bearings, what have you) a blown head gasket… or the curse of an ancient Mummy who’s eternal rest was once disturbed and who’s last remains were carried by your very vehicle!
It may be a combination of all of these things.
It may also be something else entirely.
What you need to decide is what you want the truck for. Once you do that, that will determine how you deal with it.
If you need good transportation that is reliable, I would recommend taking it to a trustworthy mechanic.
If you are seeking to tinker and fix it yourself, you need to decide how involved you want to get and what kind of investment in tools, time, frustration, elbow grease and money you are willing to make.
Do you just want to fix it up, and keep it running, or are you seeking a full restoration?
A good place to start to solve these questions and decide how to proceed is to go to Amazon.com and purchase the Chilton’s automobile guide for your particular vehicle, or the Haynes guide.
This will contain good basic maintenance and troubleshooting advice tailored for the specifics of your vehicle.
If you get into it, consider the shop manual for your particular model. This will be your Bible, and contain the information for you to do anything up to and including a total rebuild.
I hope this helps, and good luck.
Oh, and just as a point of reference, I had an old '79 Chevy automatic with a smallblock 400 and a plow I used to push snow on the farm.
One evening, I wondered why the heater never seemed to get very warm and didn’t defrost the windshield.
I tinkered around and found out the thermostat had been taken out. I found it had been removed because the radiator was in bad shape and it tended to overheat if it had a thermostat in.
So, I bought a new radiator and cooling system. While removing the old one, I saw the engine was leeking oil badly and had low compression. I knew the small block four hundred was in demand as a pulling truck power plant, so I sold the engine and bought a factory 350. I also decided that while I was changing the engine out I might as well reexamine the transmission which was slipping and I had determined the bands were to blame. Examining the scorched transmission fluid told me I needed a rebuild. Turns out it was cheaper to buy a new one, a manual one, which I preferred anyway. So I bought a manual transmission, installed a clutch pedal and found I needed new drivelines to accomodate the transmission, so I removed the bed, which I should have done earlier. While I was at it, I bought a new bed from a junkyard that was in better shape and decided I might as well beef up the axles and suspension, so I redid those, and got new tires and did a brake job. The wiring was pretty old, too, so I got a new harness and wires and conduit and replaced it all. Then I decided I might as well add a second gas tank, put in new cab and bed mounts, have the bench reupholstered and do some body work. Might as well paint and prime it too.
I did all that back in '99 and used the truck hard since without anything other than regular maintenance. Sold it about two months ago for more than I had in it.
The moral here, is that you want to think carefully before you decide you want to investigate what appears to be a small problem on an old truck. You never know what you might up getting into.
This finally led to my wife’s pronouncement that I could never buy any old equiptment ever again.