Oil Pressure Drops After High RPM

Matt69olds · Nov 27, 2024

Cut the oil filter open. If it’s clean of metallic debris, put a mechanical gauge on it to verify the oil pressure. If it’s still low, use 10W30 or 10W40. Shell Rotella 15w40 will definitely raise the idle pressure.

Having said that, thicker oil is a band aid. Something is worn to cause the lower oil pressure, the knocking sound confirms that. It might run like this a long time, it might get worse as time goes.

JamesSam · Nov 28, 2024

ChuckN said:
Can’t tell from the pic- is that residual on the dipstick and it might actually be dry?

If I’m wrong and it’s where it should be, this is what I would do if it were my truck.

If I were fairly certain that it wasn’t a knock (you’d mentioned you’d heard something) I’d hook up an actual gauge to check apples to apples for accuracy before going to a thicker oil.

If you’re worried about a knock, I’d pull the pan and make sure the oil pick up fit tightly or wasn’t clogged.

A wise man here once said that these are just indicators and having an actual mechanical gauge to verify is critical. If the gauge checks out, and your oil level isn’t low, the oiling system is dead simple. You have a pump with a pickup, a filter with a bypass mechanism in the block, oil passages and bearing clearance. That’s just about it. Well, then you have the oil pump pick up that can fall off or not fit tightly. But you get my point.

What does your spidey-sense tell you?

Thanks @ChuckN Chuck! I wish I could pick you up and take you for a ride so you could see and hear it all in person with me. My spidey-sense tells me something is off but I am not as inclined as you all with my engine knowledge. I am looking in to aftermarket OP gauges and got some 10 W30.

JamesSam · Nov 28, 2024

I have been running 5 W30 ever since I got the truck only because that is what the PO had been using. I believe I will be switching it up to thicker oil after your suggestions and my research. Thanks GMSB crew.

WFarm · Nov 28, 2024

Just for comparison, my oil pressure is at 60 PSI immediately after starting cold. On the highway after the engine is at normal operating temp it settles in the mid 50 PSI. When I stop at a stoplight when hot oil pressure drops to around 30 PSI.

This is a Mk IV 454 running Amsoil Z-Rod 10W-30 full synthetic oil.

ChuckN · Nov 28, 2024

JamesSam said:
I have been running 5 W30 ever since I got the truck only because that is what the PO had been using. I believe I will be switching it up to thicker oil after your suggestions and my research. Thanks GMSB crew.

Yeah, I get that but time for something thicker and with a high zinc content for your cam and lifters. I’ll post a picture below of a chart, but basic rule of thumb is higher than 1200 ppm
desired. Matt69olds and Ricko1966 gave you some good info and advice, I would definitely follow that.

Here is a link to the chart put out by speedway motors: https://static.speedwaymotors.com/i...3*MTczMjc5ODY0Ni4xLjAuMTczMjc5ODY3MS4zNS4wLjA.

And a lower def picture:

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Unless you’re in a very cold climate, I’d run at least 10w40, skip the 10w30. Back when I had my dad’s old 3100 with a ‘62 327, I’d just get a jug of Napa 10w40 (just rebranded Valvoline) and put some Lucas zinc additive. It never gave me any problems and it was a cheap and safe solution.

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WFarm · Nov 28, 2024

Great chart! The Speedway Motors website also discusses the Amsoil products but don’t see them on the chart. The Z-Rod line is specifically designed for older flat tappet engines:

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ChuckN · Nov 28, 2024

WFarm said:
Great chart! The Speedway Motors website also discusses the Amsoil products but don’t see them on the chart. The Z-Rod line is specifically designed for older flat tappet engines:

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Yes! That stuff looks good. I’ve never seen it locally but have wanted to run it. The local shops have started carrying VR1 synthetic so I’ve started running that- I just need to plan ahead better.

WFarm · Nov 28, 2024

@ChuckN I used to buy my Amsoil from a friend, but later just went to their website and enrolled. Now I can order from them and they FedX it here. Usually get it the next day or maybe day after. I run Amsoil in everything; cars, tractors, antique Simplity’s.

JamesSam · Nov 28, 2024

Thanks @ChuckN ! Awesome chart. @WFarm Paul, I am interested in the Amsoil Z Rod oil. Do you know if this is in any of the auto parts chains?

WFarm · Nov 28, 2024

@JamesSam I never found it in local stores. Not sure why. But you can order it off their website as I mentioned.

fast 99 · Nov 28, 2024

Have seen Lucas at local stores. Another place to check is Walmart.

OP when you drain the 5-30 out take a stick or screwdriver and swirl the oil around in the drain pan. If it looks at all like metal flake paint quit right there, bearings are in the pan.

Shorty81 · Nov 28, 2024

Also for comparison, I run Schaeffer 10-30 high zinc oil. Mark IV 496. Melling high flow oil pump. Hot idle pressure is 35lbs. Hot at 2000 rpm is 65 lbs. Normally I run 10-40 but my pressure is plenty high.

DoubleDingo · Nov 28, 2024

Back to the pics of the dipstick, cold check will show exactly where the oil level is, unless it's wiped off. Even thin oil stays put on cold check. Those pics show extremely low oil level. Change the oil to a thicker oil, change the filter, and report back.

JamesSam · Nov 28, 2024

DoubleDingo said:
Back to the pics of the dipstick, cold check will show exactly where the oil level is, unless it's wiped off. Even thin oil stays put on cold check. Those pics show extremely low oil level. Change the oil to a thicker oil, change the filter, and report back.

it's not the best pic bit if you zoom in you should see the oil on the stick. I have always been wary of the oil on one side only. It is in between both sets of dots. Is this really extremely low oil?

DoubleDingo · Nov 28, 2024

JamesSam said:
it's not the best pic bit if you zoom in you should see the oil on the stick. I have always been wary of the oil on one side only. It is in between both sets of dots. Is this really extremely low oil?

I just did a cold check on the 81. Yes, I need to change the oil, and I also see it's a half-quart low. But it covers the entire area, not just a remnant. The photos you posted it looks like remnant of oil. This cold check is popping the hood and checking the oil level without any engine rotations, and not wiping the dipstick, it shows how everything settled in and at what level since I drove it last week.

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Oil Pressure Drops After High RPM

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