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Message: <blockquote data-quote="kctam" data-source="post: 45277" data-attributes="member: 19">2cents: Good question & to be honest, am not too certain how to answer you (since am not a qualified person on this subject).I did however, do a net search & found this interesting article written by qualified people from Shell (<a href="http://www.volvoclub.org.uk/oil.htm" target="_blank">click here</a>).Permit me to quote paragraph 5 " ... We have already mentioned the polymer additive which goes to make multigrade oils. This type of additive is called a Viscosity Index Improver (VII). The term VII derives from the scale used to measure the rate at which an oil thins out when it is heated - in other words, its Viscosity Index (VI). A monograde oil would typically have a VI of 100, whilst that of a mineral based multigrade would be 150. Synthetic oils are far superior in this respect, with the typical VI of a top synthetic motor oil such as Shell Helix Ultra 5W - 40, being 187. This property is of great value in maintaining the viscosity of the oil, and therefore the protection of the engine, throughout its oil change intervals."[quote from Bob is the Oil Guy Glossary] VI is defined as arbitrary scale used to show the magnitude of viscosity changes with change in temperature. Oils with high VI number such as VI=200, will still thicken with decreasing temperature but not as rapidly, and also will thin out with increasing temperature, but again not as much as low VI oil.The deduction from this is that multigrade would retains its viscoscity during the oil change interval. Thus a monograde would be, say 50 point, initially but after usage, would change in viscosity. By how much, by thinning or thickening, when... are the questions I cant answer. Maybe someone else here can also enlighten us.2cents... why not give the monograde a try. If its not to your satisfaction, then dump it & go back to multigrade. On a personal note, I'd rather stick with modern multigrade as technology improves over the years.-------------------------------------------------------------------------------------------------To all: A point I did not stress during my previous post was that the data quoted was neither stated mineral nor synthetic. As to which is "better", that is the eternal question that will continue to be debated till the cows come home (no pun intended to our CircusCow...he he).Also the data quoted is "as is, where is" basis. That's why the emphasis was on XW-40. I would not dare to make an unqualified assumption for other multigrade that are also available (eg. XW-50). Also bear in mind that there was no mention on additives and cleaning agents found in all engine oils (both mineral & synthetic). This "holy grail" differs from product to product & that is also another reason as to why each oil "feels" different, even if the same multigrade.Thus, as in life, there are multiple factors that makes something the way it is. We cant just look at one single factor & not work the others into the equation. The same applies here. We have just looked at the (multi)grades but have not taken additives, mineral or synthetic, cost etc into the full equation.Ultimately, we all want the best for our cars & try to make the best "educated" judgement in choosing engine oils. If it makes you happy, then go for it. You are the person paying for it. So what if it differs from my opinion or someone else's. It's YOUR choice & God gave this gift of free will to you.</blockquote>

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