It is mainly an additive loss issue.  The base lubricant has some issues
too.  Being refined from crude oil, the resultant lube oil has many
different sizes of molecules in it.  The smaller ones evaporate rather
quickly and the heavier ones form deposits and sludge if left in the
engine too long.  Thus, the viscosity changes by the hour of use.  Wax
and paraffin cause the oil to thicken in cold weather and thin out when
things get hot.  Viscosity index improvers and pour point depressants
are added to counter these problems but are mechanically broken down
with use.  For 99 cents/quart, you just don't get the best additive
chemistry.
TEMiller

David Brodbeck wrote:

>On Wed, 2 Jun 2004, Doug in Calif wrote:
>
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>>Frankly, I don't buy some of the "facts" stated by others, that adding
>>synthetic oil by itself will keep your engine oil cleaner and thereby
>>allow you to drop your change intervals.
>>
>>
>
>Is it dirtiness, or viscosity change that requires dino oil to be changed
>as frequently as it does?  I've heard both claimed, in this thread.
>
>
>David Brodbeck, N8SRE
>'86 Volvo 240DL wagon
>'82 VW Vanagon Westfalia Diesel
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