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pisspodpete wrote:
Another factor is theoil's hours of running in the engine. Oil tends to shear as it is run through the internal gears & bearings, the more it shears the lower it's base viscosity becomes. Here is where synthetic oil's shine as they have a much more stable base viscosity than most Dino oil's do.Engines with internal transmissions are the worst offenders hare as they tend to shear the oil the most because of the gear teeth.
The reason you have higher oil pressure when cold is the oil is MUCH thicker when cold. As it warmsit thins out & that is why the lower oil pressure when hot. Remember 20w50 is only 20 weight oil that doesn't thin any more than 40 weight would at somewhere around 100°c.
Here is an old racers rule or thumb. You need around 10 psi of oil pressure for each 1000 RPM of engine speed under load. That usually doesn't include low idle though.
Twisty
pisspodpete wrote:
pisspodpete, HOT engine oil pressure at idle is pretty unpredictable (that's why most oil pressure readings are taken at a higher rpm). Just a couple of RPM's difference in the engine idle speed can have a big effect on oil pressure. Even oil brand can effect hot oil pressure as not all 20w50 oils have the same base viscosity (some run to the high side on theviscosity index while others are much lower).
Another factor is theoil's hours of running in the engine. Oil tends to shear as it is run through the internal gears & bearings, the more it shears the lower it's base viscosity becomes. Here is where synthetic oil's shine as they have a much more stable base viscosity than most Dino oil's do.Engines with internal transmissions are the worst offenders hare as they tend to shear the oil the most because of the gear teeth.
The reason you have higher oil pressure when cold is the oil is MUCH thicker when cold. As it warmsit thins out & that is why the lower oil pressure when hot. Remember 20w50 is only 20 weight oil that doesn't thin any more than 40 weight would at somewhere around 100°c.
Here is an old racers rule or thumb. You need around 10 psi of oil pressure for each 1000 RPM of engine speed under load. That usually doesn't include low idle though.
Twisty
