Originally Posted By: philipp10
I'm definitely no expert in oil. So your saying a 15W is more viscous than a straight 30 or 40? And what is "VII" ?
No, don't look at it that way, the 15W is just the winter rating of the oil. You can have 15w-40, 15w-50...etc. It just means the lubricant passes the cold performance requirements to carry the 15W designation.
The number AFTER the W is the number we are concerned with here, which is a 40. Now of course there are a range of viscosities that fall under the 40 umbrella, so you can have an oil that is on the thin side of a 40 or the heavy side of a 40, both still 40's, hence my European example from my earlier post.
An SAE 30 is an SAE 30, meaning its viscosity at 100C falls within the range for a 30 as defined by the SAE, same process applies with an SAE 40. A 15w-40 falls inside the same 40 range, but since it is an HDEO, it may have a higher HTHS (it is actually a bit heavier) than your SAE 40. Of course what it offers is improved cold performance over the straight-weight.
Now, VII's, they are Viscosity Index Improvers. Polymers that expand and contract that effect the operational viscosity of the fluid and improve cold flow performance (cause the oil to thicken less) and increase operational viscosity (cause the oil to thin less as it gets hot). These are used in multi-grade oils to make them meet both the cold temp viscosity target and the 100C visc target.
Where things start to get a bit sideways is when you add synthetic oils and modern base stocks to the mix. Many modern base oils (and base oil blends) have a naturally high VI (Viscosity Index) that allows them to meet both the 100C requirement AND the cold temp requirement without the addition of polymers. AMSOIL makes a 10w-30 that falls in that category, Redline's 5w-30 is another example. These products have NO VII's in them (like your SAE 30 or SAE 40) but are able to satisfy the 10W and 5W cold temp performance requirements respectively, hence being labelled as they are.
Other oils can get away with very little VII being added to the mix to meet a relatively wide visc spread (M1 0w-40 for example).
Now, VII's can (and do) shear. What this does is reduce the operational viscosity. That said, a 15w-40, by being an HDEO (Heavy Duty Engine Oil) and spec'd for high stress diesel engine applications, many of which spend a great deal of their time at wide open throttle, is not all that likely to shear. These oils tend to be very shear-stable.
The idea by an OEM to spec a narrow visc range is usually to deal with operational viscosity loss, often due to shear, fuel dilution...etc. A "normal" 10w-40 may not hold its viscosity well enough and perhaps ended up down in the bottom of the 30-grade territory in Merc's testing. This may have been why they spec'd the visc they did, which is likely guaranteed to have no VII's, so it is less likely to shear.
IMHO, a 15w-40 is still a safe bet and probably won't shear either. It may actually be a heavier oil (as I noted earlier) than the spec lubricant anyway.