Oil Viscosity/Oil Pressure

Anything's possible. . . . .but to show lower pressure with a thicker oil going throught the same size openings seems to be contrary to logic and physics. Any chance the 50wt is a multigrade? That might show a lower pressure when cold than a straight 30wt or 40wt.
 
Good point here as SAE 50 is way thicker than 20W50 cold .
Henry/Kokomo said:
Anything's possible. . . . .but to show lower pressure with a thicker oil going throught the same size openings seems to be contrary to logic and physics. Any chance the 50wt is a multigrade? That might show a lower pressure when cold than a straight 30wt or 40wt.
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here is a website that gives the reasons for oil wt ratings http://www.upmpg.com/tech_articles/motoroil_viscosity/
lots of good information. and no, 50wt should not lower your oil preasure unless there is something else wrong.
when you read into the article and you can see why 20-50 wt oils are recomended for our old fords by many people. i personally like castrol 20-50... that is a statement of choice not an endorsement for castrol.. don
 
I've seen oil pressure drop a couple of times when heavy oil is run and the oil pump screen is most of the way clogged thus restricting flow espically with heavier oil and will run low oil pressure because it is starving the pump.
Steve
 
All oil gets pretty thin when it gets warmed up to operating temperature. If I put new 100 grade SAE 50 motor oil in a helicopter, it will run near the upper red line when cold at even medium rpm then lower down to a normal reading after the oil temp is up to running temp and the engine is at full rpm. If the oil temp goes up for one reason or another, the pressure drops even more. I can't use the multi grade here in south Texas due to it thinning out to much. It can be used in the winter but not in the warm weather.
 
Professor "Old Henry" would give a more detailed answer, but this is from my old days back in the Esso Research lube oil labs. The lower number is the base oil viscosity, ie 20 weight for easy starting. the second number is determined from the viscosity increase, to the upper weight when the oil gets up to operating temperature. That is 50 weight (equivalent), is the artificial number and is due to heating the oil. The additives are called viscosity improvers, STP is pure voscosity improver. Byron.
 
Don't care much for muliviscosity oils. I use SAE 40 in the summer (to get about 5 lbs. higher oil pressure at low end) which is good down to 60 degrees starting temperature and SAE 30 in the winter which is good down to 30 degrees starting temperature. (I say "starting" temperature because that's all that really matters, i.e. if it's minus 30 outside but you can start your car in a garage that's +30 you don't need thinner than SAE 30 oil to pull out of the garage into the minus 30 and drive around. The oil will only get hotter and thinner after it starts, not colder when you pull out of the garage. So, even though it can get below 0 in the winter here, I don't need below SAE 30 oil because my garage never gets down to 30 degrees. I could run 40 all year round if I was sure my garage wouldn't get below 60 in the winter - which I'm not. Sometimes it gets down to 59.)

I stay away from multi-vis oils because of this that I read::( (Same as what Byron said)

Drawbacks of Viscosity Improving additives
Multi-grade motor oils perform a great service not being too thick at cold startup to prevent engine wear by providing more instantaneous oil flow to critical engine parts. However, there is a draw back. These additives shear back in high heat or during high shear force operation and break down causing some sludging. What's worse is once the additive begins to be depleted the motor oil no long resists thinning so now you have a thinner motor oil at 210 degrees. Your 10W-30 motor oil can easily become a 10W-20 or even a SAE 10 (10W-10) motor oil. I don't have to tell you why that is bad. The more VI (Viscosity Improver) additives the worse the problem which is why auto manufacturers decided to steer car owners away from motor oils loaded with VI additives like the 10W-40 and 20W-50 viscosities to blends that require less viscosity additives like the 5W-20, 5W-30 and 10W-30 motor oils.
The less change a motor oil has from high to low temperatures gives it a high Viscosity Index. Synthetic motor oils that are made from Group IV (4) PAO base stocks have Viscosity Indexes of more than 150 because they are manufactured to be a lubricant and don't have the paraffin that causes the thickening as they cool. But petroleum based motor oils (Group I (1) & II (2)) usually have Viscosity Indexes of less than 140 because they tend to thicken more at the colder temperature due to the paraffin despite the addition of Viscosity Improving additives. The higher the Viscosity Index number the less thinning and thickening the motor oil has. In other words, high number good, low number bad. Low numbers thicken more as they cool and thin more hot. You see these Viscosity Index ratings posted on data sheets of motor oils provided by the manufacturer.
Because stress loads on multi viscosity motor oils can also cause thinning many racers choose to use a straight weight petroleum racing motor oil or a PAO based Synthetic which do not have the VI additives. But only the Group IV (4) PAO based synthetics don't need VI additives. (Read more here: http://www.upmpg.com/tech_articles/motoroil_viscosity/)
 
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40 Deluxe said:
Please don't say "weight" or "wt" when discussing oils. "W" stands for "WINTER"!!!
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A "W" after a number in multi-viscosity oil is the oil's viscosity at 0 deg. F. So 10W-30 means it acts like SAE 10 at 0 degrees and SAE 30 at 210 deg. Although "weight" is not really descriptive of viscosity it is the most common nickname. (The technical article I refer to actually uses that term in conjunction with the word "straight" to designate non-multi-viscosity oil.) The true designation for viscosity is "SAE". (I revised my post so as not to offend any sensitive eyes.)

Read more here: http://www.upmpg.com/tech_articles/motoroil_viscosity/
 
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I remember when muti viscosity oils were introduced, I could never figure how an oil the looked & felt like SAE 10 oil could have a viscosity of SAE 40. The key to this explanation deals with oils at different temperatures. If you could (without burning youself) check the feel of an SAE 40 oil when it is at 200 degrees if will feel like SAE 10 does cold or even perhaps thinner. The temperature expanding polymers insure that a warm oil will act like a cold oil does so it will function as a lubricant between those temperature extremes. The only thing I havn't figured out now is exactly how long those polymers will last before they degrade beyond the point of normal function. Most figure around 3000 miles and some say more now with more efficient fuel injected engines but I'm skeptical about how much more.
 
currently i have a 2009 ford F150 3 valve engine with 6 speed auto trans with a 9500# tow package. just returned from 7200 mile trip towing 7000# 5th wheel rv. truck has 46k miles. use mobil 1 synthetic 5w20 and mobil 1 filter. consistantly change at 3500 miles and Northside Ford service supports those intervals even when towing. in my 50 car with a new chev crate motor, after the break in period with mobil non synthetic 30w and Crane break in additive, i now use mobil 1 10w30 high milage that has increased zink addative and change at 3000 mi or 3 months, which ever first.
i had no problem useing mobil 1 15w50 year round in all my low and high milage flatheads. just changed all cars at 2500miles. even used te mobil 1 15w50 in the 50s OD transmissions.

this AMSOIL website provides lots of information on automotive oils and describes multi viscosity oils http://www.upmpg.com/tech_articles/motoroil_viscosity/
 
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