01 May 12 15:48, Mark Hofmann wrote to Roy Witt:



 RW>> If you're using a multi-weight oil, think viscosity. i.e. with
 RW>> 10w-30, think of the 10w viscosity as winter flow. Today's
 RW>> multi-weight oils are tested for flow at controlled tempuratures,
 RW>> the 10w meaning that the oil will flow like a 10 weight oil at a
 RW>> cold temp such as is encountered in the winter time.
 RW>>
 RW>> The 30 in that nomenclature means that the oil will flow (viscosity)
 RW>> like a 30 weight oil at a higher temp. So, the warmer 10w-30 oil
 RW>> gets, the closer to the viscosity of a 30 weight oil, rather than
 RW>> that of a 10 weight oil.
 RW>>
 RW>> When you park the car overnight, the warm engine temp will keep that
 RW>> leaky lifter from leaking down for a longer period of time,
 RW>> hopefully keeping its 30 weight or heavier viscosity longer. If its
 RW>> a garage kept vehicle, it may not get cold enough to thin out the
 RW>> viscosity and keep the lifter pumped.

 MH> So the oil is actually thicker at hot temperatures or when it is hot
 MH> outside if you are using a 10w 30?  10w for cold and 30 for hot?

The oil may not be, but the polymer additives they use shrink or expand at
those temps changing the viscosity along the way.

As you suggest, pure engine oil will thin out at higher temps.

I can varify that by a story my dad used to tell about his earliest
experiences with cars in the 1920s. In the winter they would build a fire
under the engine's oil pan to thin out the oil before they 'hand cranked'
the engine to start it. Of course, that was before multi-weight oil was
invented.


                R\%/itt


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