Dumb Question: Heat and Oil Pressure

MustangPaul · Aug 26, 2004

Dumb question: Will higher temps--both external and engine--make your oil pressure dip a bit or will the oil pump hold the pressure steady no matter the temp.

Reason that I ask is that my Oil PRessure was down a tad today. It is 98 with heat index of 114 and I was running with the AC on--raising the engine temp a bit.

Just wondering.

--Paul

WORTH · Aug 26, 2004

MustangPaul said:
Dumb question: Will higher temps--both external and engine--make your oil pressure dip a bit or will the oil pump hold the pressure steady no matter the temp.

Reason that I ask is that my Oil PRessure was down a tad today. It is 98 with heat index of 114 and I was running with the AC on--raising the engine temp a bit.

Just wondering.

--Paul

YEP, hot oil is thinner and makes less pressure.

Max Power · Aug 26, 2004

Not only that, your parts expand and normally tight oil passages have loosened up a bit too. The reason I am rebuilding my engine is because I was losing oil pressure on hot days.

MustangPaul · Aug 26, 2004

Good to know. Max, how low was your oil pressure dropping?

Thanks, all!

Ozsum2 · Aug 26, 2004

So, is the viscosity rating just a wives tale?

Max Power · Aug 27, 2004

Below 5psi. The motor was old though.

While a 20w-50 rating is a measure of protection, I don't think it is a measure of flow characteristics. I could be wrong, but over the years I have used thicker and thicker oil, and in the end, 20w-50 with STP added still wasn't enough.

Just driving around town was fine, but on a hot day after driving a few miles on the freeway, I would really have a problem at idle.

WORTH · Aug 27, 2004

Ozsum2 said:
So, is the viscosity rating just a wives tale?

To be honest I have no f'n idea. 10/40 is suppose to be thin when cold, but it never acts that way. Just pour it out of the bottle when it's cold and you'll see.

Ozsum2 · Aug 27, 2004

Max Power said:
Below 5psi. The motor was old though.

While a 20w-50 rating is a measure of protection, I don't think it is a measure of flow characteristics. I could be wrong, but over the years I have used thicker and thicker oil, and in the end, 20w-50 with STP added still wasn't enough.

Just driving around town was fine, but on a hot day after driving a few miles on the freeway, I would really have a problem at idle.

It's supposed to be a marker for flow. The thicker the oil, the more oil pressure, but with less flow. The thinner the oil, less pressure but more flow. It does work that way, but on a cold day, worth is right, the thin oil is thicker than snot.

MustangPaul · Aug 27, 2004

So what do you guys run? I'm running 10/40 Havoline. Should I go to 20/50 or a straight 30?

Ozsum2 · Aug 27, 2004

MustangPaul said:
So what do you guys run? I'm running 10/40 Havoline. Should I go to 20/50 or a straight 30?

I run 5w30 Valvoline in the 67, and Mobile 1 5w30 synthetic in th '04 Buick

bull999999 · Aug 27, 2004

Max Power said:
Below 5psi. The motor was old though.

I'm getting a little bit over 20psi after the motor's warmed up with 10W30 at 800 rpm, with a little over 40psi at speeds. Is that high enough?

Ozsum2 · Aug 27, 2004

bull999999 said:
I'm getting a little bit over 20psi after the motor's warmed up with 10W30 at 800 rpm, with a little over 40psi at speeds. Is that high enough?

Yea, that's fine. If you go below 20, I'd switch to a 10w40.

Max Power · Aug 27, 2004

Not only does oil get thicker when it gets cold, I believe it gets a lot thinner when hot.

50w oil at 200 degrees will pour a lot faster than 10w oil at zero.

That's why I always thought viscosity ratings measured protection more than flow, although I think flow is a factor.

There are so many wives tales about oil these days, especially concerning synthetics, that I don't know what to believe anymore.

All I know is, the hotter the car got, the less oil pressure I had, even with 50 weight. It was always fine cold though.

I s'pose I could do a Google and bone up on viscosity again, but I am off to California on business. Maybe I'll have time when I get back.

Ozsum2 · Aug 27, 2004

Max Power said:
Not only does oil get thicker when it gets cold, I believe it gets a lot thinner when hot.

50w oil at 200 degrees will pour a lot faster than 10w oil at zero.

That's why I always thought viscosity ratings measured protection more than flow, although I think flow is a factor.

There are so many wives tales about oil these days, especially concerning synthetics, that I don't know what to believe anymore.

All I know is, the hotter the car got, the less oil pressure I had, even with 50 weight. It was always fine cold though.

I s'pose I could do a Google and bone up on viscosity again, but I am off to California on business. Maybe I'll have time when I get back.

The question is................is 50w thicker than 30 when they are hot? Probably so.

MustangPaul · Aug 27, 2004

so 5/30 in cold weather and 10/40 in warm?

Ozsum2 · Aug 27, 2004

MustangPaul said:
so 5/30 in cold weather and 10/40 in warm?

Yep. I run 5w30 year round because both of my engines are in great shape, and I like the extra miles per gallon on fuel.

MustangPaul · Aug 27, 2004

Oz, plus it gets PLENTY hot down here.

When did choosing an oil get so damn difficult? What about the "High Mileage" oils?

I get a lot of engine noise when I run 20/50, so I don't run that anymore.

The stuff we run in my wife's Diesel VW is delvac-1. Pure snythetic at $25 a gallon. Of course, it lasts about 10,000 miles. Still . . .

--P

Ozsum2 · Aug 27, 2004

Maybe just run a straight 30 weight then. You really don't need the benefit of the 5 or 10w during start up.

gt350dave · Mar 13, 2006

oil viscosity #'s

For some good info on oil, check out this link: http://www.advanceautoparts.com/english/youcan/html/ccr/ccr20010101ov.html It gets pretty technical, but is good info. Later...

66HertzClone · Mar 14, 2006

Here are two interesting articles to read, those with flat tappet cams should pay attention to the New England Classic article.

http://www.newenglandclassics.org/Articles/The Evolution of Engine Oil.pdf

http://www.bobistheoilguy.com/

Dumb Question: Heat and Oil Pressure

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