Is a Higher Viscosity Index for oil good or bad?

I am trying to decide on a Synthetic15W-40 Oil for a Diesel Engine. RedLine and Amsoil seem to be the two best, but I'm not sure which is better. Based on these stats, which of the two oils would be better for a diesel engine?





AMSOIL Kinematic Viscosity @ 100掳C, cSt= 14.4


RedLine Kinematic Viscosity @ 100掳C, cSt= 14.5


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AMSOIL Kinematic Viscosity @ 40掳C, cSt= 89.7


RedLine Kinematic Viscosity @ 40掳C, cSt= 97.0


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AMSOIL Viscosity Index= 167


RedLine Viscosity Index= 155


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AMSOIL Pour Point 掳F= -44


RedLine Pour Point 掳F= -49


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AMSOIL Flash Point 掳F= 453


RedLine Flash Point 掳F= 486


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I'm not sure if higher or low is better when it comes to these stats. Could you please help me with understanding these? Thanks.Is a Higher Viscosity Index for oil good or bad?
use the redline, more viscosity is always better. the viscosity is how slippery it is. i see that the amsoil has the superior viscosity to red line,but the redline provides more protection in all other fields,so itll be better for your engine.have you checked into shell rotella-t? or lucas oil stabilizer? flashpoint is what temp it needs to be to catch on fire. so naturally higher temp is better,means it'll take longer to catch on fire. pour point is how cold it can be and the fluid will still flow. the -49 is colder than -44 so itll give you 5 degrees colder protectionIs a Higher Viscosity Index for oil good or bad?
rotella 15w40 is the only way to go with diesels.
Higher is bad period. You dont want to go over a 30W for any reason. I use 5W30 in the winter and 10W30 in the summer.
Mark's got it right. The zinc additive is an anti-scuff/anti wear agent. It was removed from gasoline engine oil last year, as it can degrade catalytic converter life by a smidgen. Several cam companies suggest a zinc oil or a zinc additive to prevent cam lobe and lifter scuffing with flat-tappet cams. Most new cars have gone to roller cams since the late '80s, so its not ';needed'; as bad for newer gas engines.





Right now, diesel rated oils still have zinc content. They may take it out next year, as low-sulfer deisels with converters come into the market. Don't know how that is going to affect diesel engine life, as diesels get lugged and worked hard at low RPM.





I add a pint of the STP oil treatment with every oil change in my Bel Air and GTO. Everybody thinks STP is just thick goop. But if you look at the corner of the front label, you'll see ';ZDDP anti wear agent';. Zinc diphosphate, baby!
Higher viscosity for hot weather, lower for cold. It's the external temp. that matters.
I would have to agree with theleb63, and I kinda agree with phillip t. The RedLine does look a little better as far as the numbers are concerned, but you won't notice too much of a difference between the two. Philip t suggests Shell Rotella. I used that oil in my 18 wheeler and had not a single problem with it as long as the oil changes were kept at regular intervals. I don't know if the Rotella has a synthetic oil on the market or not. It wasn't available at the time I used it, but synthetic oil wasn't that big a deal yet. As far as diesel engines are concerned, a 15W-40 oil is ideal. Here's a few links to get you started. The first is for the Rotella T regular oil. notice the specs compared to the synthetic oils that you listed. The second is a direct link to the Shell website, but I didn't see a 15W40 full synthetic there. Good luck!


http://kozmik.guelph.on.ca/gtdproject/fl鈥?/a>





http://kozmik.guelph.on.ca/gtdproject/fl鈥?/a>
I wouldn't get excited about the difference. Changing oil is much more important than getting the perfect oil. If you pay too much for the oil you may be reluctant to change it as often. A well worn engine may be helped by a higher viscosity. Low temps outside a lower viscosity.
This whole issue depends on climate, driving conditions, and motor wear.





The colder it is, the thinner you need it (lower visc)


Hard driving needs a thicker oil, and so does engines with lots of wear.


Out of your list the RedLine looks better overall / just remember Diesel's get oil fuel polution faster than a gas eng will, they are also harder on oil overall.





(Note: The comment about high visc oil being bad is simply wrong - try driving that motor with 10w30 in death valley and see what happens)
Is either oil a diesel rated oil? Gas motor oils do not contain the correct additives to properly protect a diesel motor. I recently read an article (either in Car Craft or Hot Rod), that recommends diesel oils for NEW or rebuilt motors (especially high performance motors) at least for the break in period, due to the extra protection offered by the diesel oils. The additives are minor but add slightly to emissions in gas engines.





If both oils are diesel rated, looking at the specs you posted, you are really splitting hairs on this subject. Both oils are extremely good and I would be most likely to use the one that you have the best access to. (dealer close by or good friend deals the product)





Mark in NE Indiana