Low oil pressure after going to synthetic

I have a '93 K1500 4x4 w/5.8l.
At my last oil change, I decided to try out the Castrol synthetic. It seems to run fine, but sometimes (usually at stop lights with the A/C on) the oil
gauge drops to nothing and the engine light comes on. If I kick off the A/C, the RPM's jump just enough to bring the oil pressure up to a minimum.
Does this sound like a failing oil pump, or like I just need to go back to standard SAE-50 due to old age. The odometer just flipped over to 158k miles.
Thanx- Derek
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What weight syntec are you using?
As a side - I've got about 370,000 on my '79 K1500 5.7, and run 10-40 in it with oil pressure hovering around 15 psi at idle, so maybe 50 wt is a bit thick?
Jeremy

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15lbs of oil pressure????? Yeah, yeah, I know, at idle. Is this a diesel?
Hatt
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(not a top post this time, although I find it easier to read the replies at the top without having to scroll all the way through sometimes tedious, long-winded messages)
Nope. Not a diesel. Just a good ol' 350 4-bolt. Keep in mind tho that I run 10-40 oil in it, even though it has 370,000-something miles on it.
At crusing speeds, it keeps the gauge pegged when cold, and when warm it holds about 30-45. When it starts dropping, I know it's time to change the oil again. :-)
Jeremy

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I agree with you there sir. But, it makes it harder for anyone else who wants to get in on the thread to reply to it in a consistent readable form. I'm just relating experience since beginning posting over 4 years ago. Usenet has changed alot since then, [used to have more respect for convention], but its still usenet and there is a good reason why its supposed to be done that way. They've been doing it a lot longer than we have.

OK, I was just shocked at the low oil pressure. I do know that high oil pressure can indicate "bad" things too.

Mine, a 2.8V6, with less than 3 thousand miles on it, [rebuilt], in an S-10 holds 60lbs, warm engine thats been on the highway for a half hour. 10w40 @ 60 degree outdoor temp.
Hatt
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eh, sometimes things need to change.. ;-)

I would think 10-40 would be too thick for a freshly rebuilt engine - the bearing tolerances are still too tight to get enough oil squeezed in there when cold........

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Well, I'm just doing what I was told. Same as anyone else who chooses to listen to a dealer, or manufacturer. We all get to choose based on what info we have, I hope.
Hatt
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I just tossed an 85 S-Blazer with 2.8 on the scrap pile, three new engines in less than 70,000 miles. Body was mint but I just hate the 2.8 in the blazer, low power, tow rating a VW Bug can match and passing power equal to a slug on teflon...
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I'd guess the problem with your 2.8 wasn't something that could be blamed on the oil, but thats just a guess of course. I drive like a grandpa, not only because I AM a grandpa, but because I know this 2.8 is not a power wagon. Wasn't meant to be, but it gets me anywhere I want to go. I've had them before and never had a single problem with one, till I bought this one used. But thats probably because I changed the oil more frequent than recommended. Whoever had it obviously didn't take care of it, much less change the oil. I wish you would have told me before you got rid of those 3 new engines! Damn, can't win em all can we?
Hatt
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inline....

a 350 is a 5.7l...

what weight oil did you have in it and what weight of syntec did you put in?

pressure sender first. in my '88 K2500 w/ a 350 if I have a high electrical load (lights, wipers, blower on high, etc.) the OP warning light will flicker at idle. however, I also have a mechanical OP gauge installed in an A pillar. while the stock gauge is dipping towards 0 and the light's flickering, the mech. gauge shows a rock solid 10psi at a hot idle (400RPM). (which is well within spec).
Basically hook up a mechanical gauge and watch it was the truck warms up (before you suspect the oil pump/bearings).... my haynes book says min is 6psi/1000RPM and 18PSI/2000RPM... conventional wisdom is 7psi idle and 10psi per 1000RPM.
HTH, Bret

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Oh, yeah, the 5.8 was a fat-finger mistake.
Yes, I'm using 20w50.
Thanks for the idea. I'll just replace the sender and see if that changes anything. Everything else I've replaced on this truck was original, so I'm sure the oil sender is!
Thanks, -Derek

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As a warning (if the sender is mounted next to the distributor) the senders are mounted in a brass 90deg fitting you have to be very careful when unscrewing the sender if you put *any* force on the fitting it can break off on you (been there done that) which will cost you a couple hundred to get it drilled out and retap the hole.... Just be careful...
mark
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Cool, thanks for the tip! That would be the start of a really bad night...
-Derek

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Derek,
I run regular Castrol GTX 20-50 for years. I have always found Castrol makes a very good oil. I did however try their syntec in the same weight and found a lose of 10 Psi. I found this to be normal after much reading and a call to Castrol. General consensus was this was normal given that it is actually so much thinner and is so slick. I ran it for 3,000 miles which is my regular maintenance schedule and change it back to my staple of GTX. It was a personnel choice and when I put in a new motor next year I will run a synthetics after the first five hundred miles. Both oils are great, but keep in mind if you have any leaks in the oil system, it will show up.
Regards, Chuck 78' Chevy 1 Ton 4X4 Van
: I have a '93 K1500 4x4 w/5.8l. : : At my last oil change, I decided to try out the Castrol synthetic. It seems : to run fine, but sometimes (usually at stop lights with the A/C on) the oil : gauge drops to nothing and the engine light comes on. If I kick off the : A/C, the RPM's jump just enough to bring the oil pressure up to a minimum. : : Does this sound like a failing oil pump, or like I just need to go back to : standard SAE-50 due to old age. The odometer just flipped over to 158k : miles. : : Thanx- : Derek : : :
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Chuck, that's good to hear, and that's exactly what I was guessing. I bought the 10w40 synthetic and it seems to really thin out when the engine heats up. When it's cold, it stays right around 30 psi. When the engine is hot, and I'm running the A/C (yes, it's still hot here in Arizona, even at night), and I'm in drive (looks like about 450 RPM), the needle will sometimes drop all the way down to the the 0 mark. It could be what one of the other guys said: an old, worn out pressure sensor. Maybe the synthetic just drops the pressure too low for the old sensor to read, but a new one might be able to read it.
Still, it's scary to see the pressure gauge needle just drop to 0 and not move! I'm probably going to make the switch back to regular oil, like you did. This old engine runs smoothly and doesn't blow any smoke. I was told that when the time comes to retire it, I can get a new crated engine from a Chevy dealership for about $1,400 with a 3-year warranty. I'll probably just run this one on regular oil until it dies and then do what you suggested about switching to synthetic on the new engine after break-in.
Thanks for the reply! -Derek

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