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Message: <blockquote data-quote="kctam" data-source="post: 45093" data-attributes="member: 19">2cents: Yeah.. "cold" start here is not that cold but as most of the oil has gravitate to the sump, the faster the oil flows through during the start phase, the better. This is where the balance between "cold" start protection vs. high rev protection comes into play. Read below & see if it makes sense.Errr.... I'm no expert on oils & am sure someone here may have the proper technical background to answer this professionally. Will however try but bear in mind that I'm not 100% right on this.Here goes.... from a company's P&L prespective, a multi grade oil was designed primarily for the "mass" market (ie. USA). As the US (or "mass" market) has such wide range of climate (from artic to tropical), from a profit & loss point of view, it makes cents to make ONE or TWO product that would suit all these conditions (less $$$ wasted in R&D, production & stock keeping).From a (un qualified) technical point of view, a straight 50 point oil would take  longer time to reach it's "operating" temp & during this (warm up) period, the wear on the engine would be more significant than a multi grade oil (which has better "cold" viscosity). If I'm not mistaken, the viscosity of a straight 50 point oil is worse (ie. thicker) than a multigrade, hence the engine would be struggling to overcome this 'treacle'. A document I saw recently has interesting data. For 0W-40, 5W-40, 10W-40, 15W40, 20W-40 & 25W-40, the low shear rate kinematics (at 100C) was the same, however, the high shear rate (at 150C) was better / higher for 15W-40, 20W40 & 25W-40. The conclusion I draw from this was, if you want higher temp protection, for the same (upper) grade (ie. XW-40), choose one that has a higher winter grade (ie. the XW is 15W or 20W or 25W) (I noticed 20W & 25W are very difficult to find nowadays).Interesting note is that the high end shear kinematics (at 150C) for 15W 20W 25W - 40 is the same as a single 50 point grade oil. Maybe that's why a multigrade can function as well as a single grade oil (at high temp).Another point to note is that the lastest (2004) API-SM is almost the same requirements as the ILSAC GF-4, which is signed by Japanese Motor Manufacturers, GM, Ford & DaimlerChrysler. Basically API-SM / GF-4 oils will have to meet certain wear & tear, corrosion, volatility, enviormentally friendly requirements, amongs them lower emission. Thus, the new grade of oil would probably be no higher than XW-30 as listed in the document (with XW limited to 0w, 5W & 10W). Though I suspect a XW-40 multigrade may be available (if it meets the standards). Whats the point I'm making? Future multigrade oils will probably have lower 'high' end (40 or even 30 point), if they want to meet API-SM / GF-4 or higher requirements. Hence, start looking at the "winter" grade (ie XW) to help you decide which grade suits you best.So, a 0W to 10W grades will probably give better "cold" start protection but a 15W to 25W grades will give better high rev protection (for a XW-40 multigrade oil "apple to apple" comparison).Ultimately, the choice is yours & yours alone. The decision is yours to make.Disclaimer: The opinion above is not from a qualified oil or chemistry engineer, thus any conclusion or judgement was based on 'layman logical' understanding. The author bears no responsibilities (morally or physically) to your car and engine should you agree to his conclusion (ie. at your own risk).</blockquote>

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