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Originally posted by Buster:
Consensus is less VII's the better. I don't think anyone would dispute that. Are there any reasons where VII's are ideal even in high performance applications?

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what is it thats bad about VII's?

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Originally posted by PetrochemUK:
G'Day - I stumbled across this topic doing some research in another field, and thought that I could shed some light on the situation.

Modern day viscosity improver additives function very differently to how they did at thier genesis, when there was a lot of scepticism surrounding teir use. A Viscosity improver doesn't just make the engine oil thicker, it acts to even out the logarithmic relationship between viscosity and temperature (as the temp increases a little, the fluid thins out a huge amount).

The current use of soluable polymer type viscosity improvers aims to rectify several old problems. Firstly, at cold temperatures, modern Viscosity Improvers are in tightly bunched balls which leave the oil thin and allows good cold-cranking indices. As the oil heats up, these polymers 'untangle' and expand to create a large increase in volume without an increase in weight, and as such imporves the viscosity.

Old VII used to have the problem of being 'torn apart' or decomposing in high pressure shear-stresses found around surface interfaces(leaving waxy, polymerised deposits), however this problem has been overcome by the design of a 3D type 'star' shaped polymer (similar in appearance and action to a tumble weed) which will tumble along the interfaces when exposed to shear stress, rather than decomposing.

As remmious has correctly stated, VII's are designed to decrease engine wear over large temperature variations, and aim to have excellent cold-cranking properties, as well as excellent high-shear-rate viscosities, which leads to more consistant performance. Unfortuneately, by virtue of their make-up premium base oils can never hope to achieve the performance consistency of modern day VII's.

On another note, synthetic oils remove all hetero-atoms found in mineral oils (even premium ones), particulatly sulphur, which is a huge contributor to sheared-asperic corrosion and hence, engine wear.