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A single conversation with a wise man is better than ten years of study.
"When injustice becomes law, resistance becomes a duty."
1985 28' Regal
I lifted this from my response on another forum regarding STP (one of the very early VIIs):
"Polyisobutylene is a petroleum product; the stickiness is a characteristic of the polymer and of the cheap oil used to dilute it. It was one of the first viscosity index improvers (VII).
The VI of an oil is an index of its resistance to thinning as the temperature increases. A typical straight SAE 10W oil would be "off the charts" thin when measured at the higher summer-grade temperature. A typical straight SAE 30 would be "off the charts" thick if measured at the winter-grade temperature.
So a 15W/40 oil is one with VIIs added to a 15W base stock, such that at the summer grade temps it still hasn't thinned out below SAE 40.
Basically, an oil's viscosity is a function of its molecule's size, especially length. Asphalt has very long molecules; those of pentane, benzene, heptane, and octane (gasoline) are short.
A VII is made up of molecules that at the winter grade temp, are coiled up, so act like short molecules. As the temperature rises, they uncoil and act like longer molecules, resisting thinning.
The problem with polyisobutylene is that the molecules when uncoiled were mechanically sheared into smaller bits, and the resistance to thinning was destroyed. Worse, those remaining pieces tended to become ring belt and valve deposits, and worse still, could become hard and gritty.
Now, VIIs have been much improved, to where they're "shear stable", and that's why you'll sometimes see a multigrade motor oil referred to as 'shear stable'."
Anyway, DD finally decided to place the blame for the ring belt and valve deposits onto ash content >1.0% after mandating single weight and having the problems persist. However, the primitive VIIs caused their shar of deposits. Now multigrade oils are not only approved but recommended, at least in the 4-stroke. As I mentioned above, single weights are the default choice unless some unusual circumstances dictate a multiweight.
There is no reason not to use multiweight - the recommendations for engines like MR's were made in the '70s and '80s, long before shear stable VIIs were concocted. Neither the engine builders nor oil blenders are going to go back and redo the tests to upgrade the recommendations.
Rotella 30 is excellent, but very hard to find; Rotella T 30 is better, and also hard to find. IMHO, spending time and miles hunting either (or Mobil Delvac 30) is a waste of time and gaso. If it's available easily, get it.
'94 28' Breakaway: MilSpec AMG 6.5L TD 230HP
Nelson and Chester, not-spoiled Golden Retrievers
Sometimes I think we're alone in the universe, and sometimes I think we're not.
In either case the idea is quite staggering. - Arthur C. Clarke
It was a woman who drove me to drink, and I've been searching thirty years to find her and thank her - W. C. Fields
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