Oil pressure quote

[Rod2]

Rick Popely of cars.com was quoted in the Charlotte paper March 24, saying: "Thicker oil....reduces oil circulation around the engine and increases oil pressure, which means there will be more pressure trying to push the oil past seals and gaskets."
What do you think of his remark? I've used 10/40 all my truck's life, and, yes, I have oil leaks, but I thought that thicker oil would have a better chance of staying in.
School me.

[az chip]

That is not how any of this works.

[SemperFIGuy]

So let's think about this logically, with a question, what is the FSM recommended pressure at idle with a warm engine for a 360?

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[Lumpskie]

That quote doesn't make sense to me when I think about multi-viscosity oils. A 10-30 and 10-40 will be the same "thickness" when warm... which resembles a 10 weight oil.

[tgreese]

Well, no. The last number is the viscosity rating at 212F, the temperature of boiling water. So a 10W40 will have the same viscosity as single-grade 40 at 212F. The first number is the behavior of the oil in winter (W), so a 10W40 will have the viscosity of 10 wt single grade at 0F. This is still thicker than the same oil at boiling temperature ... multivis changes the amount of viscosity change with temperature. Any oil will decrease in viscosity as the engine temperature rises ... but multivis changes less than single grade.

The reasons you run thick oil in a worn engine are 1) the oil is thicker, so it's less likely to be squeezed out of the rod bearings in response to the piston movement (this is the big reason). Clearances in worn engines are large, and the larger gap lets the bearings drain faster under the force of the rod. 2) hydraulic lifter leak down will be slower, making a tick less likely. 3) splash will drain more slowly from the cylinder walls, improving the ring sealing. Hopefully less oil will go past the rings and down the valve guides.

That the oil pressure goes up when you run thicker oil is just a side effect of the slower drainage from the bearings. I believe the oil pressure is out of the picture once the cylinder fires, and whatever film is in the bearings is all there is to prevent metal-to-metal contact, until the down stroke is complete. Then the oil pressure can restore the film.

[Lumpskie]

Thanks for the clarification Tgreese. I did get those numbers backwards.

[fulsizjeep]

So let's think about this logically, with a question, what is the FSM recommended pressure at idle with a warm engine for a 360?

1977 TSM indicates 13 psi at warm idle.

[Stuka]

Read this (all chapters), and you will understand oil: https://bobistheoilguy.com/motor-oil-101/

[letank]

which means there will be more pressure trying to push the oil past seals and gaskets."

I may not be totally correct, the way I see it: Oil is not really pushed at seals and gaskets... except at the oil pump... most of our leaks are "splashes from rotational motion and vapors" unless you have some blocage at the PCV splash shield...

There is a some pressure in the engine due to the motion of the cylinders but the venting thru the PCV and the oil fill tube alleviate pressure built up inside the engine... I can feel the air pressure when I put my hand over the breather tube of the single cylinder motorcycle... but it is a lot less on the V8 as cylinders are going down, others are going up, the dollar bill test...

As for oil pressure, it is always a dilemma of pressure vs. flow, pressure maintains a certain flow, but higher flow with less pressure helps cooling...

[Nikkormat]

Oil always take the path of least resistance. Thats the thing to remember.

Oil technology has advanced alot since the 70's when our oil system was designed. But our engines were designed for 10w30 none the less. I would say that was just to be competitive with Chevy the same as specing only 5qts of oil but we may never know for sure.

I've run a lot of different weights in the Cherokee's 401 (which has about 160k on the crank, no machine work done there) and the sweet spot seems to be 5w40. That offers plenty of oiling to the valve train and plenty to the rods and mains. And it flows better cold than 10w30.

Running thinner oil you loose more to the valve train. With a 10w30 you will loose around 20 psi hot idle. Because the oil travels up the pushrod into the head instead of down the oil gallery into the mains and rods. This can be observed by pulling the valve covers while it's running and comparing the amount of oil exiting the pushrods.

Our oil pumps are simple mechanical devices. As long as you aren't activating the pressure regulator in the filter adaptor the displacement is fixed. Which means you have the same amount of "flow" at 20 psi as you do at 50psi (which is also why I disagree with the whole high pressure spring is bad dogma too but I digress...). There is a small variation because of resistance but it isn't enough to starve a bearing or make a difference to anyone but the most astute hydraulic nerds.

For increased crank oiling with thinner oil there are pushrods available with a smaller oiling orifice. Bulltear offers them here: http://www.bulltear.com/catalog/product ... cts_id=156

Some people say thinner oil is cheaper, but when you factor in the cost of a zinc additive I say the cost difference is negligible between gas engine 10w30 and diesel 5w40. And the diesel oil has more detergents, anti oxidizing compounds, and is designed to be better at high load lubrication than a simple zinc additive.

Just my 2$'s worth.


But that's not to say you can't go too thick with your oil too and starve the valve train and cam gear etc... for oil. The 15w40 people run is fine for warm weather but I say it doesn't flow well enough cold for use in places that it gets below 30°f.

[candymancan]

My 360 runs with 10-30 at 10-15 psi hot in Gear idle and about 50 psi driving and 30 psi hot idle in park.

With 10-40 it runs at 20-25 psi hot in gear idle, eith 60 psi driving and 40 psi hot in park idle.

I decided to buy 10-30 oil with stp for zinc and thickening oil. Engines runs a lot better at least to me with thicker oil. I sometimes put 10-40 in with rislone zinc instead. Has the same affect on oil pressure i noticed.

I thought i had a rod knock but when i dropped my pan a week or two ago and removed bearing caps 7-8 thr bearings looked brand new and the rod journals didnt any if very little scoring couldnt feel a thing on my nails. So i think these engines like thicker oil tbh.

I used stp in my 5.9 zj and noticed oil consumption was cut in half.. went from 1 quart every 3k miles to about half a quart