Oil pressure drops to zero

1988 K1500, 350ci EFI When the engine gets warmed up, after about 3-5 miles of driving, the oil pressure will drop to zero when I come to a stop with the trans in
gear. Putting the trans in neutral or revving the engine will increase the oil pressure. I've also noticed the oil pressure gauge is reading lower than it used to since the last oil/filter change. Now while driving and fully warmed up the gauge reads at most 30 psi, used to read 40-45 psi.
This engine does not burn much oil at all, maybe a quart/3K miles. Oil looks "normal" for an engine with 200K+. I.e., starts out looking like fresh oil and progresses to dark brown, never foamy or with air bubbles, no bad odors.
Previous owner replaced main bearings at about 160K. (He saved receipts and I have them) Are they shot again? Or is this something else?
Truck has had 33x9.5 tires for "a long time". Would running at low RPM (because of the larger diameter tires) in OD cause this?
Since this started I have not driven the truck much and always put the trans in neutral when coming to a stop.
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On Sun, 13 Aug 2006 03:04:58 -0500, Kevin Snodgrass

FIrst thing you need to confirm oil pressure with a manual gage attached to engine and find out if it is low. Next stop using 5w30 if you are using it and it is a very poor choice in a high mileage engine. Use 10w30 minimum here and maybe 15w40 in hot weather (not 10w40) Also, you current gearing and tire size is hard on tranny and engine because lugging it at lower RPMs under load like that is hard on bearings over time. Look in your glove box and see what axle code is in there (GU6, GT4 or GT5) ----------------- TheSnoMan.com
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Hi SnoMan,
Sorry about not getting back to you sooner but I haven't been outside much lately. Even though it's much cooler lately it's still to hot for me to be outside when the sun is up. :-)
SnoMan wrote:

Not sure when I can get it checked with a manual guage. I don't have one and I looked up the location of the sender, looks like something I'm not thrilled about trying to replace myself. Besides the 33x9.5 tires this truck also has 6" body lift, I need a step ladder to get to the sender.
Oil is Valvoline 10w30. I'm in Omaha, NE and it has been quite hot recently, but mostly mid 80's the last few days. Why do you suggest 15w40 and not 10w40? Seems like the progession would be 10w30 -> 10w40 -> 15w40 -> 20wsomething.> engine because lugging it at lower RPMs under load like that is hard

GT4
Is there someplace I can find out what all those 3 letter/digit codes stand for?
I've been thinking about getting a different vehicle anyhow. This old truck has many more problems than low oil pressure (broken windshield, rear window is loose, no A/C, no stereo, can't unlock the doors with the key, muffler rusted out, etc...) There's a really nice looking Super Bee for sale nearby. :-) But I'll probably get another 4x4, either Chevy or Dodge, maybe a diesel instead of gas.
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...
Like everything else, go to google
GQ1 : AXLE, STD RATIO GS1 : AXLE REAR, 2.73 RATIO GS3 : AXLE REAR, 3.73 RATIO GS4 : AXLE REAR, 3.70 RATIO GS5 : AXLE REAR, 4.11 RATIO GS6 : AXLE REAR, 4.56 RATIO GT1 : AXLE REAR, 2.56 RATIO GT2 : AXLE REAR, 2.29 RATIO GT4 : AXLE REAR, 3.73 RATIO (DUP WITH 5 X 1) GT5 : AXLE REAR, 4.10 RATIO (DUP WITH GT8) GT7 : AXLE REAR, 3.33 RATIO GT8 : AXLE REAR, 4.10 RATIO (DUP WITH GT5) GTY : AXLE, WIDE TRACK GU1 : AXLE REAR, 2.41 RATIO GU2 : AXLE REAR, 2.73 RATIO
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On Wed, 16 Aug 2006 02:34:33 -0500, Kevin Snodgrass

10w40 should be avoided completely. GM had boat loaded of stuck piston ring issues with 10w40 in 80's from it.10w40 has a very high VI content (a Viscosity Improver) that has been added to oil to increase its stabilty accros a wider temp range. VI does nothing to improve lube qualities of oil, it actually detracts from it and has a tendance to cook out in rings. 15w40 has a lower VI content and also is a much heavier duty spec oil because it is diesel and gas rated.

THis one is easy, it is a 3.73 axle ratio which is really too tale for 33 on that truck. I have put together a list of GM axle ratio code and you can find it at the following link
http://forum.snoman.com/viewtopic.php?t

I would advise against a diesel for serious off road work for a few reasons. The first is weight. It adds several hundred pounds to front axle and weight is the last thing you want to add for hard core offroad not only because of added component stress but also extra weight to anchor you in the mud with. The second reason is a wide RPM range is desirable when you want to wind it up and "clean the mud of the tires" when searching for a grip which a good strong gas motor does well and lastly while water injestion is bad for any engine it will take a LOT less of it to destroy a diesel motor than a gas one if you suck up a bit of water. ----------------- TheSnoMan.com
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