Oil shear

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Nov 21, 2007
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Once an oil shears will it continue to shear the longer it is left in. I hear a 5w30 will shear to a 20w after say 3000 miles. Will it keep shearing to a 10w if left in too long? This also has me wondering about what will happen in a shared sump motorcycle engine. Thanks
 
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 Originally Posted By: mjoekingz28
This also has me wondering about what will happen in a shared sump motorcycle engine. Thanks
It's not pretty. If you leave it in long enough it will continue to shear till the VII's are exhausted would be my guess. When I had my two early 80's Suzuki's at about 1200-1300 miles the clutch started acting funky when fully warmed up. Grabby engagement is the first sign. In shared powertrains I would not past 1500 miles in hot weather. Your clutch will thank you.
 
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It all depends on why its shearing. If it due to engine design and it sees high heat(if its the saturn in your sig then I would say no) or if its due to fuel or coolant. It will shear more if you are driving short distances to a certain point but if you leave it long enough it actually thicken and can cause sludge. You just have to find your sweet spot OCI's
 

JAG

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Permanent mechanically-induced shearing of the polymeric viscosity index improvers is a self-limiting process. That means once they get broken (which usually happens in roughly equal sized parts) those parts are much more difficult to break up again. There is good discussion of this in at least one oil-related book. That's where I learned this.
 
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As JAG says, it isn't the oil that shears. It is the bits of polymer that straighten when cold and curl when warm so the so-called multi-grade oil flows more slowly through the orifice @ 100°C and appears to be a higher viscosity grade. (Or do they curl when cold and straighten when warm???) In any case, better quality/higher cost viscosity index improver polymers don't shear in normal usage. This is one advantage of the better oils. One of the requirements for the oil specs, and getting tougher each spec, is the "aging" test where the oil is re-tested after 100 hours in the test engine and the drop in viscosity, and other variables, gets tighter limits each spec. The up-coming GF-5 spec will be the toughest yet. As mechtech2 says, the oil thickens when it oxidizes, but that type of thickening does not protect the engine the way properly thick new oil protects.
 
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