Please move this thread if needed, couldn't find how to post it in 'normal' Gen 2 section
Here's the deets:
1999 5.9LDodge 1500 4x4
202,000 miles
New to me 2 1/2 years ago
New radiator and water pump
Farm use mostly, maximum 25 miles at a time use on pavement maybe 2-3 a month
Pennzoil dino 10w30 oil changes - Temps here can reach 0 degrees winter (no usage of truck or very little then, and highs of 120 in summer)
Previous owner used Mobile 1 dino 10w30, no high mileage
Napa Gold quality filter
Rarely need to top oil off tho under heavier use am sure I would, Prob need to replace some gaskets but no oil spots underneath or fresh ooze coating metal
NO CEL no oil pressure warning lights coming on. Cluster works, all other gauges obviously working, one light currently on for windshield wiper fluid
Prior owner said 80ish cold start was 'normal' for oil pressure and under rpms with some fluctuations tp 40's idling/stepping on and off gas. That has been the case so far no issue and predictable. It's always been on the high side as 'normal' but does hover around 40 depending..

The weather just turned really hot and I used the AC on one of my short 10 mile runs. I noticed the oil pressure falling (more, not normal) as I took foot off the gas to turn into my destination. Pulling in with slow acceleration the oil pressure gauge went up over the 40 midline a bit but idling to stop it just kept falling enough to be alarming. I watched this again carefully driving towards town afterwards and stopped to fill up the tank. It was steadily dropping by the time was pulling up to the gas pump with below 30's but never zero. Turned AC off, filled, drove a short one mile to get out and walk dog, no AC. Upon starting up after 1/2 hour 45 minutes the oil pressure gauge started out in the 'normal' 80 range for this truck with drops down to below 30 as I drove home (AC off).
I drove it again today NO AC 10 miles round trip and saw the gauge start out at 'normal' pressure, then drop down to 30's and 20's. Doesn't sound 'bad' exactly but I've never seen this much fluctuation with a gauge needle heading left and down in over two years. Cue the freakout and am now neck deep in researching 'engine flushes' and how best to very slowly clean up what am suspecting could be engine crud in an oil screen, or worse. Will pull out a code reader tomorrow for chits n giggles tho no lights on.

My concerns are this truck gets little 'normal' mileage use in a very dusty high wind environment, temperatures can fluctuate in summer from 120 in the day to 60's at night. Top speeds are 55-60 for short duration with norm being about 45 mph.
My oil level is correct, no odd colors or smells, oil is clean but hitting change interval. I also live at least 40 plus miles (one way) from any type of service centers/mechanics, I do my own oil changes on my vehicles, Pan drop appears complicated according to Haynes manual, no way I can do that, not my expertise nor have the equipment.

Besides questioning could AC use mess with my oil pressure gauge (never did before), I've decided I need to somehow do a very gentle cleansing and dissolving of any possible gunk clogging lines or screens up. If anyone has ideas, feel free.

I've read tons of threads here and there's the debate over switching to synthetics on high mileage second owner vehicles (potentially loosening whoop ****) versus simply using something like seafoam in a dino 10w30 and doing quick oil changes in a row. My problem being I simply don't drive this truck enough to add significant mileage to it and def not interested in starving n coking metal by continuing to drive it while doing nothing proactive to try and determine what is going on and problem solve . Not sure if my ridiculously low mileage usage is a factor in what may or may not be a budding oil clogging problem in my engine. It's been running just fine with regular non eventful oil changes (tho spaced further apart). I hear no lifter tick noises, not sure what's going on.
My ideas:
A. Seafoam or similar added with a good enough dino oil. Depending on result, do two or more oil / filter changes in a row, finish with a high mileage dino (or synthetic)
B. Run high mileage Amsoil through it (synthetic) with added miles driven, change and inspect oil for signs of definite color change (and look for leaks) - proceed from there
C. Let some mechanic have a field day with the lady and go on a wild goose chase dropping the pan etc.
Since I have no CEL light on now, I'm more inclined to trust my own sleuthing and problem solving over driving in to a shop and playing the 'car repairs while being female' game. Trust me, 'we' need to educate ourselves as best we can before handing our vehicles over. Learned that a long time ago.

 

I think you're over analyzing the situation. If pressure dropped enough to be concerning the check gauges light/chime would go off. One reason cars went to on/off type gauges. Seafoam would only thin out the oil resulting in lower pressure. It would probably do a little cleaning if you wanted to run it for a a trip or two and then change the oil. If you're level is good, good color. I'd change it with you normal oil you've always used and continue on. The pan is very simple to drop on these. Take the bolts out from the block, remove trans supports and done. Might take a little wiggling and twisting to pass the suction tube but nothing extreme. Engine definitely has some high miles. Could try switching to a 15W-40 oil to bump pressure a little bit, or even a 10W-40. Could also be an issue with the oil pressure sensor.

 

Have you tried shutting the ac off when you see the pressure drop? Does the gauge go back up? Also does the rpm of the motor drop/raise any? If you are that concerned about it i would put on a mechanical gauge. The stock gauges are not that accurate and give odd readings. My volt gauge will read correctly one day and the next low. Voltage was confirmed by a meter and it's wrong. You can also do the dash test and see if there is any issue with the gauge.

 

Hey thanks DT2! So I don't need a chain hoist to get the pan dropped? Haynes seems stuck on hoists for the 5.9L V8! I'll get under there and study, would love to get the pan n screen looked at.. I'm only using it 4x out in the pasture to cut juniper n haul limbs to the landfill so wasn't too worried about the high miles. The trans and 4x was pretty new rebuild when I bought. Would you bump up on oil weight and switch to synthetic or leave well enough alone?
Oh yes I have been accused of 'overthinking' things haahahah but since I live out in the middle of nowhere alone with closest neighbor a mile away, no cell service after I leave my wifi satellite connection, chit can get really real really quick haha. AAA ain't 'a phone call away' and you might not see one other driver in a stretch of 6 hours on some of these roads. Add the occasional wolf and cougar sighting and you can sharpen a pucker in a nano second. Staying alive if broken down on the side of the road can become a thing wrong season wrong road. Let's just say I'm 'pre-emptive' on the thinking part before chit goes South bwaha!

 

Well that was a fun little test, thanks! No issues with the gauges, nice to see the needles moving as should be. No I didn't immediately turn the AC off while driving to watch the needle, will take a spin today and try. One thing that came to mind this morning was I did feel/hear like a break in the AC blowing hard on 'Max AC' when I was accelerating up a fairly long grade but not terribly steep. I just don't know enough about these trucks to figure out how one system can slop over n monkey wrench another. Only clue I've got is that and prob unrelated to the oil pressure gauge. Just something else to fix hahahahahaha !

 

I don't know how far you are from any NAPA or similar but ideally you'd verify the oil pressure with a mechanical gauge which you often can rent there. I've never seen the oil pressure go all the way up to the 80 mark on a cold start (granted small sample size) so it could be a faulty pressure sensor.

Obviously running the AC increases the engine load and with the condenser sitting in front of the radiator engine temps might slightly rise which could contribute to lower oil pressure. Having said that I just drove my 5.9 gas in 100F temps with the AC on full blast and both temp and oil pressure didn't look any different from cooler days.

 

Well I finally got a short drive in but not duplicated exactly as before.
Tried AC on AC off, no change in oil pressure so that was just a coincidence.
Cold start is 80'ish-90 (normal for this truck), warmed up coasting, foot off gas/foot on brake / idle in park or D is 30 (which is the 'not normal' for THIS truck since I've had it. I did see the needle slightly wavering at 30 back and forth very slightly when I was in park with motor up to temp. I know for most people they'd say mmm 30 psi is normal, but it's not for this truck and it had dipped lower than 30. This drive it never once dipped below 30 but I need to drive it longer to duplicate, bad heatwave up to 106-7 so not inclined to go cruising.
I listened to the engine with AC maxed and max fan on and it was noisy, actually sounded like there was an air leak somewhere sucking haha. Also checked my snorkel fit to airbox and it seems a bit jinky loose with play for my tastes where it makes contact with the air filter housing. I got way too much flying grit, sand, fine powdery dust blowing for that.
Decided to go ahead and change out the oil after running Rislone Maximum Performance engine treatment. Not gonna leave it in there tho that's what they say to do. Just don't drive enough even for the 'run it double bottled for 100 miles, then change oil' if engine extra dirty. Dunno that it is at this point.
I'm gonna start at a baseline of new dipstick, new oil, new filter, clean the throttle body intake, eventually take entire thing off and put new gasket, check/replace sensors, drop oil pan off check screen, once heatwave passes try my luck driving over 100 miles rt to a Napa or O'reilly's.
Only thing 'different' in my oil pressure falling the first time besides it's never gone that low in the few years I've owned it, is, a) it's super hot out, b) due to my 'severe conditions' heat, dust blah blah n short trips, the oil I have in right now could be shot. No frothing, no weird colors on oil cap, radiator fluid is fine and overflow level correct. Maybe it's just n tired thinned out tho looks 'clean' and my gauge is reflecting that. Highly doubt it.
Could an oil pump go out 'intermittently' or do they usually crap out all at once and you only know it when the light comes on?
With my pressure seemingly high at cold start compared to other Rams, would bumping up to a higher 40 weight oil really be a good thing? Decided not to run to a synthetic blend either. It was born a dino truck, it will stay dino, not messing with it. Until I get to a Napa or something will make sure it's got fresh oil and filter. About all I can do right now besides try and hook a finger or allen wrench into the oil pan bottom and rotate around in there at oil change, see what comes out of that fishing expedition.