Just one of the things unique to Dexos spec.
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The GM dexos oil has one spec that ILSAC and API labeled oils do not have. This is quite important! That dexos requirement is for better resistance to aeration, which is the whipping of air bubbles into the oil. Engines with variable valvetrain (variable camshaft timing) technology use engine oil as a hydraulic fluid to move components in the engine, i.e., hydraulic lifters.
If air bubbles are in the oil, the lifters will not act as fast as the engine controllers expect. On the mild end, this can limit engine performance and economy. On the extreme end, this slow response can throw a MIL or DTC check engine code. Chevy, Dodge and Ford police vehicles all use engines with a variable valvetrain timing!
For all practical purposes, put Mobil 1 in all GM police vehicles. The use of synthetic oil has always been a good match for how we use our vehicles. Now, according to GM, it is a requirement.
http://www.hendonpub.com/police_fleet_manager/articles/2012/1112/dexos1_motor_oil_required
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Key differences between dexos1™ and ILSAC GF-5/API SN are:
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GM Proprietary Test: Opel RNT and Opel OP-1
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Includes ACEA Engine Tests: TU5JP, TU3MS and M271SL
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Higher Seq. IIIG Weighted Piston Demerit requirement: 4.5 vs. 4.0
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Higher Seq. VG sludge requirements: AES 8.3 vs 8.0, RCS 8.5 vs 8.3
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Lower Noack Volatility: 13% vs 15%
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Better Shear Stability: Requires passing the Bosch Injector
Shear Stability Test
http://www.mfaoil.com/images/E0138901/dexos.pdf
And Dexos vs GF-5
http://www.pceo.com/101510_EO_PV1202_AR.pdf