Castrol Syntec 5W50 best 50?

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Feb 17, 2004
If one wants the lowest possible viscosity at start up, then why would one ever choose any 15W-50 (such as M1) over Castrol Syntec 5W-50?
Generally speaking, a 15w50 is going to hold it's viscosity better than a 5w50, as it will use less viscosity index improvers.
Castrol Syntec is not the only 5w50 in North America John, Quaker State also makes a 5w50. And I could be wrong but I thought Pennzoil did too.
Patman thanks!!! I forgot about QS makeing a 5W50. I have not seen it much. I think I saw it once at K-Mart and that is it! None of the other stores in my area carry QS 5W50! So until recently the only widely available 5W50OTC oil was Syntec 5W50. Sorry for all the typo's in my previous post! The group II should read group III and the 0W50 should read 0W30!
What makes you think everyone wants the lowest viscosity for starting? Ambient temp plays a huge role in this decisions so does shear stability and purpose. If you want a 50Wt. then usualy cold cranking and pumping are not a real issue for you! 50Wt. is used to keep a certain viscosity at the bearing when you engine is running realy hot due to rpm and load in high ambient temps! When you start to make the viscosity spread to wide you end up with a thin oil with a lot of thickners in it "VI's". VI's do nothing to luberiate the engine. So the more Vi's you have the less oil and anti-wear "AW" additives you have in the oil. If you push an oil that is loaded with VI's and a weak additive package it will begin to fail early on! M1 15W50 is probably the best OTC 50Wt you can buy for preventing wear under extreme conditions! THe only OTC 5W50 in the USA is Castrol Syntec. It is pretty well acepted that it is mostly group III (likely aroung 60%-70%) with some Group IV/V oil as well. The thickest a group three can be made with out VI's is a 30Wt. A 30Wt group II likely would not be able to get 5Wt rateing though. ANytime you try to have a wide VI spread you end up with a very comprimised product. Now just because it is comprimised does not mean it will not protect well. It means it will not do as good of a job as more specialized product. A full synthetic 0W50 or 5W30 is going to cran easier and pump easier in extreme cold then a 5W50. A 15W40,15W50 or 20W50 is going to out perform a 5W50 at extrem loading and extrem shearing especialy in high ambient temps with high engine oil temps! I suspect that 5W50 is a good comprimise for your average joe. Their is a reason though that no manufacture has recomened or branded Castrols 5W50 as their own. Put 15W50 in and drive it for 5000-12,000 miles and do a UOA on it. Then do the same with Castrol 5W50. I think the differences should become aparent. I am testing Castrol 5W50 in my wifes car right now. Seeing how it is cooler out right now I would do 5W50 first to take advantage of it lower pump and flow points! THen do 15W50 as it warms up! [ February 29, 2004, 12:19 PM: Message edited by: JohnBrowning ]
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