oil viscosity

joe_mnAugust 29, 2012

i saw some 0w-40 oil for sale. for a newer VW? why do some car brands use such high viscosity oil? our new camry uses 0w-20. yay, the zero weight is for low friction at startup but why is the hot vis so high? i thought bearing clearances were designed to accept low vis oil for better mileage?

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0W - 20

Viscosity of oils is a function of temperature. in general, as the temperature of the oil increases, its viscosity decreases. This was true of older oils made in the 1940 - 1950s. Those oils, referred to as straight weight did not have viscosity improvers and was mostly straight oil with maybe a few additives.

Viscosity improvements were found and a new designation used such as 10W40. This meant that the oil had the viscosity of a 10W oil when cold and did not thin as much as straight oils when heated. When hot, a 10W40 oil has the viscosity of a straight 40W oil.

One manufacturer argued that the 10W40 oil had too much modifiers in it and it was loosing "lubricity" when hot. It began recommending 10W30 oils instead. Over the years while this oil discussion and various developments took place, engine head operating temperatures had climbed to as much as 260F with the norm being 195 to 240 F.

Fits of sleeve type bearings were tightened and end leakage out if the bearing was reduced. This permittted the oil pump to maintain a healthy pressure with hot oil which permitted the use of thinner oils. Many older cars (10 yrs old and more) used 10W30 in hot weather and 5W20 in winter. The recommended oil for my 2005 Honda is 5W20 the year around. A change to a different oil is recommended only for extreme cold or extreme hot climates.

Some writers state the the "W" in 5W20 refers to "winter".

Whatever the case, the best policy is to use the oil weight recommended by the manufacturer. This recommendation is based on hours of engine durability testing by the engine maker.

For sleeve type beaings, besides temperature and vicosity, the two principle details that affect oil flow out of a bearing is bearing clearance of the load bearing surfaces, and the end clearance. When end clearance open up, oil leaves the bearing at a faster rate and if excessive, lowers the oil pressure delivered by the oil pump.

Many automotive engines use a fixed displacement oil pump and a pressure relief valve. Under all conditions, the pump should deliver an excess of oil with the excess vented off by the relief valve. With a worn engine and hot oil, the delivery may become marginal to the extent that the relief valve no longer opens due to low pressure. Of course, oil squirted out the relief valve is a loss and that energy goes to heating the oil. The desgin goal is to minimize this loss without jepordizing the engine under hot conditions. Closing up bearing clearance reduces the volume of required oil flow.

    Bookmark   August 31, 2012 at 2:30AM
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the camry says use 0w-20. which is syn oil. do you think using 5w-20 dino oil would have an effect on cold startup wear? how about 5w-20 syn oil? just for discussion. yes, syn 5w-20 costs the same as syn 0w-20.

    Bookmark   August 31, 2012 at 10:57AM
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