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Discussion Starter · #1 ·
The all confusing question of exactly what the oil grades mean and the thickness of engine oil...

I have a 1995 Rodeo 3.2L with 210k miles. I am having an issue keeping my oil pressure up once it gets warm. It will start at approx 45psi when cold startup, but drop to near 5psi once it gets warm. I was running 10W 30 oil with a bit of Lucas oil stabalizer for a while but decided to change the oil and run a straight grade of Rotella SAE 30. My reason for this is, I dont want my oil to change grades and get thinner once it warms up in attempt to keep my oil pressure up. Im aware that it is possible that the engine could need a major overhaul because of the oil pressure issue, but the expense is beyond me at the moment so I need to do something to keep the oil pressure up.

Is there anyone that can explain how oil grades work, its really confusing to me. All I do know is that no matter what oil grade you run, it is going to be thicker at cold startup and get thinner at operating temps.

Am I ok running a straight grade oil instead of a multi grade oil? btw...I live in Dallas, where the climate is warm/hot alot.
 

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http://www.upmpg.com/tech_articles/motoroil_viscosity/

is a good article. Executive summary is a 5w-30 behaves like a 5w oil when it is cold and like a 30w oil when it is hot.

So when you changed from 10w-30 to 30w you were unlikely to see any increase in hot oil pressure.

Given your climate (much like ours in OKC) I would look at Rotella T6 which is a 5w-40 synthetic or if you are cheap look at Rotella which is a 15w-40 conventional. As hot as it is in the summer you might be able to get by with a 20w-50 in the summer - I know I ran that for a while in one of the 95 Troopers I had.
 

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No engine oil gets thicker as it gets warmer. That is not possible.

The first number in a multi grade oil IS NOT A VISCOSITY RATING. It is a cold crank rating. Do not compare this number to the second number, which is the viscosity rating.

So comparing a 5W30 to a 10W30 the 5W30 will be flow better when cold and allow the engine to crank easier. This is good for engines that live a life below freezing. However once at full operating temp both oils have the same viscosity. So this will not band aid your low pressure.

Now if you take a 10W40 and 10W30 they will both behave the same when cold but when warm the 10W40 will be thicker and have a higher viscosity. This will help increase your hot oil pressure.

When my 2.8L had trouble keeping hot oil pressure I ran 10W40 or 15W40 oil and it worked well.

The 30 weight oil you bought will be very thick when cold, thicker than a 15W40. However it will be similar to 10W30 or 5W30 when hot. Typical straight grade oils typically do not have the additive package and detergents that you find in multi grade oils and should not be used for long periods of time. They also do not have as stable of a viscosity. The viscosity swings more with temperature change. The only engine I know of that specs single grade oil is a Detroit Diesel.
 

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vanduker said:
Executive summary is a 5w-30 behaves like a 5w oil when it is cold and like a 30w oil when it is hot.
No, it meets the standards for 5W when it's cold, and it meets the standards for 30 when it's hot.

I think the dumbest thing the SAE ever did was give oil grades numeric names, and then they compounded it by putting the "winter" grades in the same "scale".
 

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Thanks for the corrected info. Short version though is the OP ought to try a 10w-40, 15w-40, 5w-40 or something along those lines.
 

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betterthanyou said:
No engine oil gets thicker as it gets warmer. That is not possible.

The first number in a multi grade oil IS NOT A VISCOSITY RATING. It is a cold crank rating. Do not compare this number to the second number, which is the viscosity rating.

So comparing a 5W30 to a 10W30 the 5W30 will be flow better when cold and allow the engine to crank easier. This is good for engines that live a life below freezing. However once at full operating temp both oils have the same viscosity. So this will not band aid your low pressure.

Now if you take a 10W40 and 10W30 they will both behave the same when cold but when warm the 10W40 will be thicker and have a higher viscosity. This will help increase your hot oil pressure.

When my 2.8L had trouble keeping hot oil pressure I ran 10W40 or 15W40 oil and it worked well.

The 30 weight oil you bought will be very thick when cold, thicker than a 15W40. However it will be similar to 10W30 or 5W30 when hot. Typical straight grade oils typically do not have the additive package and detergents that you find in multi grade oils and should not be used for long periods of time. They also do not have as stable of a viscosity. The viscosity swings more with temperature change. The only engine I know of that specs single grade oil is a Detroit Diesel.
the vdub motor in my sand rail calls for a single grade as well..
OP, in short, the higher the numbers you get, the thicker the oil is going to be :)
 

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vzfox said:
The all confusing question of exactly what the oil grades mean and the thickness of engine oil...

I have a 1995 Rodeo 3.2L with 210k miles. I am having an issue keeping my oil pressure up once it gets warm. It will start at approx 45psi when cold startup, but drop to near 5psi once it gets warm. I was running 10W 30 oil with a bit of Lucas oil stabalizer for a while but decided to change the oil and run a straight grade of Rotella SAE 30. My reason for this is, I dont want my oil to change grades and get thinner once it warms up in attempt to keep my oil pressure up. Im aware that it is possible that the engine could need a major overhaul because of the oil pressure issue, but the expense is beyond me at the moment so I need to do something to keep the oil pressure up.

Is there anyone that can explain how oil grades work, its really confusing to me. All I do know is that no matter what oil grade you run, it is going to be thicker at cold startup and get thinner at operating temps.

Am I ok running a straight grade oil instead of a multi grade oil? btw...I live in Dallas, where the climate is warm/hot alot.
I would suggest figuring out what the problem is and actually fixing that. All your doing by trying to put a band aid on it is prolonging the inevitable. That low oil pressure is going to destroy your engine. If you find the problem and fix it,then your engine will run until something ELSE kills it.
 

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Discussion Starter · #9 ·
wow, lots of respones...thank u all alot

and yes, alot of u were right...changing my oil to a straight grade 30 hasnt helped my hot oil pressure issue. I will probably change my oil soon to the suggested 15W 40 while its summer time and prob change to 5W 40 once it cools off outside.

mallen....u are correct, my engine probably does need a rebuild of sorts but my bank account wont allow it :(

As for the post from Quezzy....this is my first isuzu vehicle and i bought it used, so I really have no idea if my gauge is inaccurate. The sending unit costs around $130 for my rodeo, for me to spend that money just on a "guess" that my gauge is wrong is not something im willing to do at the moment. If I knew for a fact the sender for my gauge was bad, I would buy it. Can these gauge sending units be taken off and soaked or cleaned somehow, since they are quite pricey?
 

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vzfox said:
I will probably change my oil soon to the suggested 15W 40 while its summer time and prob change to 5W 40 once it cools off outside.
Why bother? They'll behave the same when hot.

vzfox said:
The sending unit costs around $130 for my rodeo
I think your estimate is about $100 high.

You can test the gauge and sender if you like, but really it's easier just to replace the sender.
 

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$49.99 at RockAuto... check Jerry's price on an original, though.

5W will flow better when it's not 100 degrees... but they're going to flow nearly the same when it is, so just run 5W year-round.
 

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Apeiron said:
vzfox said:
I will probably change my oil soon to the suggested 15W 40 while its summer time and prob change to 5W 40 once it cools off outside.
Why bother? They'll behave the same when hot.
I should clarify; 15w40 really won't do anything to raise your oil pressure, if it does it's only very little. I run that because diesel oils have more detergents and additives in them so hopefully I can squeek a few extra miles out of it before I have to rebuild/replace!
 

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Discussion Starter · #14 ·
vzfox said:
The all confusing question of exactly what the oil grades mean and the thickness of engine oil...
fourzeedee1 said:
Apeiron said:
vzfox said:
I will probably change my oil soon to the suggested 15W 40 while its summer time and prob change to 5W 40 once it cools off outside.
Why bother? They'll behave the same when hot.
I should clarify; 15w40 really won't do anything to raise your oil pressure, if it does it's only very little. I run that because diesel oils have more detergents and additives in them so hopefully I can squeek a few extra miles out of it before I have to rebuild/replace!
Im confused again :(
 

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betterthanyou said:
No engine oil gets thicker as it gets warmer. That is not possible.

The first number in a multi grade oil IS NOT A VISCOSITY RATING. It is a cold crank rating. Do not compare this number to the second number, which is the viscosity rating.

So comparing a 5W30 to a 10W30 the 5W30 will be flow better when cold and allow the engine to crank easier. This is good for engines that live a life below freezing. However once at full operating temp both oils have the same viscosity. So this will not band aid your low pressure.

Now if you take a 10W40 and 10W30 they will both behave the same when cold but when warm the 10W40 will be thicker and have a higher viscosity. This will help increase your hot oil pressure.

When my 2.8L had trouble keeping hot oil pressure I ran 10W40 or 15W40 oil and it worked well.

The 30 weight oil you bought will be very thick when cold, thicker than a 15W40. However it will be similar to 10W30 or 5W30 when hot. Typical straight grade oils typically do not have the additive package and detergents that you find in multi grade oils and should not be used for long periods of time. They also do not have as stable of a viscosity. The viscosity swings more with temperature change. The only engine I know of that specs single grade oil is a Detroit Diesel.
Motor oil is composed of polymers. When cold their molecules resemble a compressed spring and when hot they stretch and look like a stretched spring. This provides a lower viscosity when cold and higher when hot. :D
 

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Sorry, not trying to confuse you!

I think the first thing you should do is get an oil pressure gauge that you can attached directly to the motor to get a more accurate reading. That way you can tell if your in-dash gauge is messing with you or not.
 

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At 173 degrees F(or something close to that, I forget what the standard is, the oil you put in will be weight 30. Anything hotter than the standard and the oil will be thinner, anything colder, the oil will be thicker. this is true the oil you put in because it is a flat weight 30. THe problem with this is that in the cold the weight 30 oil would be too thick to lubricate the engine when you started it in the morning or from any cold start. And with the engine sitting for hours most of the oil would have dripped to the oil pan leaving little to lubricate the parts. The oil that would eventually get there would be A. too thick, so it would not have the correct properties to lubricate properly and B. it would take a lot longer to reach to the critical points. This is why in the old days you'd run thinner oil in the winter, because flat weight oils is all they had.

So you're thinking "well they ran thinner oil to make up for it, whats the problem?" Well the SAE 20 oil that you'd put in the winter is rated weight 20 at 173 degrees. So although the oil will be thinner on the cold start than SAE 30 which is a good thing, once the engine got warm your oil would also be thinner than the SAE 30 which would not be ideal.

Once the technology got there multi-weight oils were made which caused for a single oil around the year to be used. For the most part most people need and use 5w-30. But if you live in colder areas 0w-30 might be necessary. This is how I think of it.

Some numberW- some number.

Some numberW I think of as "some number winter", and "some number" alone I think of as the normal number. In other words, when it's 32 degrees out side, which means it's probably winter your oil will have the same thickness as some number W. If the number is 5 then at 32 degrees F it has the same viscosity as SAE 5 oil. As the engine gets warm and reaches that normal operating temp of 173F the oil also slowly gets thicker until at 173 F it has the same viscosity as SAE 30 oil. So with 15w-40 oil at 32 degrees the oil will have the same viscousity as SAE 15 oil and at 173 degrees it's SAE 40. The point of this is to get oil to the critical parts of the engine fast, and thinner oil will travel faster than thicker oil. You will probably be fine with SAE 30 oil for now, but before it gets too cold get 5w-30.

And the way the multi-weight oils work is that there are polymers in the oil that will expand when heated, and when heat is taken away they shrink letting the actual oil flow around more freely. There are also die hard people who run flat weight oils because they say those polymers in the oil take up space of where oil would regularly be, thus youre losing some lubrication. Though this is technically true the "extra" wear is so insignificant that it's not even worth thinking about. They are probably OCD, though I can understand when racers do it.
 

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Med!c said:
And the way the multi-weight oils work is that there are polymers in the oil that will expand when heated, and when heat is taken away they shrink letting the actual oil flow around more freely. There are also die hard people who run flat weight oils because they say those polymers in the oil take up space of where oil would regularly be, thus youre losing some lubrication. Though this is technically true the "extra" wear is so insignificant that it's not even worth thinking about. They are probably OCD, though I can understand when racers do it.
I can understand why racers do it because their requirements are very very different. Most of their wear comes during a race as a incredibly tweaked engine puts out ridiculous amounts of power and sometimes the motors are rebuilt after a single race. Obviously most of the wear is occurring when the engine is hot.
On the other hand,Ive been told that a significant amount of the wear in a normal engine occurs when starting it. The multigrade oil is made to flow properly when the engine is cold as well as when its hot so that it can protect the engine during startup when a large part of its wear occurs as well as when it is running.

My feeling is that I trust the engineers that built the car. They usually specify multigrade oils so I use what they recommend. They didnt just grab a bottle off the sheld and decide thats what to use. They took into account the clearances,the viscosity of the oil,the thickness of the oil film between parts and the load that oil film had to support to prevent metal on metal contact,both during startup and operation. They decided to recomend certain oils based on that. I tend to think its absurd to think that I know better than that,unless there is some very specific issue at stake.(for instance,some Isuzus burn oil and we have learned that certain oils work well to minimize that. The engineers actually screwed up made some bad assumptions. Unless I know something like that though,I wont second guess them)
 
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