An American article on engine sludge, Power flushing etc.
www.andrewsofprinceton.com/shared/nines/002.cfm
I know oil change intervals vary by operating conditions but once again I note the often recommended US period is much shorter than our 6K /6 months.
from the link....
"Most professionals in the auto-repair industry would suggest 3-5,000 miles regardless of the type of oil used and not go more than 4 months regardless of miles."
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Synthetic oil molecules are stronger than mineral molecules and are more "shear stable."
Oxidation occurs when an oxygen molecule replaces one or more hydrogen or carbon bonding sites, resulting in peroxides, causing sludge. Mineral oils are less stable than are synthetics.
The very large Viscosity Index Improving additive molecules are more prone to shearing than are oil molecules, mainly because of their molecular arrangement and size. These are more prone to "permanent" shear, leaving the large unattached molecules to thicken the oil over time. If heated and oxidized, these remnants also contribute to sludge. Synthetics require very few viscosity improvers compared to mineral oils.
The more shear stable oils are necessary for the longer oil change intervals and these are now specified by MB etc. Prevention of sludge is better than any cure and high quality oils help the prevention of sludge.
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