What goes into a 15W40?

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Nov 6, 2003
Akron, OH
I remember reading somewhere on here that a mass-market conventional 5W30 starts out as a straight 20 WT base oil, then gets doped up with Viscosity Improvers (VIs) and pour point depressants. This treatment allows oil that wasn't 5W30 by nature to behave as if it is in fact 5W30.
Neat. Now, what goes into a normal, mass-market, non-synthetic, non-synthetic-blend 15W40?
How about a 10W30? A 20W50?
The FAQs on the European Redline site has a discussion of this. They state that a straight 30 oil will meet the 20w test criteria and so could effectively be called a 20w30. Then by adding viscosity index improvers a 20w50 was created. I would suspect a 10w40 and maybe 10w30 are both based on an oil that would fit in the SAE 20 weight category, but a 5w30 must start with a thinner oil--maybe SAE 10

What goes into the 15w40 is a real puzzler.

I wish each multigrade would list the 100C viscosity of the base oil used in it prior to adding viscosity index improvers. Then we would have some idea how robust the base oil is.

[ July 02, 2004, 04:07 PM: Message edited by: TallPaul ]
None of the above.

Look at this chart of base oils to make the new GF-4 engine oils. It doesn't include 15W-40, but you can read between the lines.

Use of different base oil Groups are possible, and different base oil viscosities, if the suitable additive package is used.
Note that the designations for the base oils vary. 100R, 220R, & 600R represent the viscosity in Saybolt Universal Seconds @ 100°F. 5R represents the viscosity in centiStokes at 100°C. For example, 20 wt. has a viscosity of 5.6 to 9.29 cS @ 100°C, in the range of 200-300 SUS @ 100°F.

Keep in mind that a 220R base oil is NOT a 20 wt. engine oil...but very similar viscosity.


[ July 02, 2004, 05:07 PM: Message edited by: Ken2 ]
Thanks Ken2. All else equal the base oil should be thicker in a 20w50 and should get progressively thinner as we go down the viscosity scale: 15w40, 10w40, 10w30, and 5w30. But all else is not equal. It appears that companies use more lower group oil in the narrow spread oils (20w50, 10w30).

Also there is a lot of leeway to meet the spec for a given grade. For example, a 10w30 must ve between 9.3 and 12.5 cSt viscosity at 100C, must have a minimum HT/HS of 2.9, must have a max cranking viscosity at -25C of 7000, and must have a max pumping viscosity (with no yield stress) at -30C of 60,000. This leaves some leeway as to how to acheive the grade by varying the base oil viscosity, base oil quality (group), pour point additive, viscosity index improver additive, and other factors.

So the question of wht makes up a 15w40, or any other grade, can only be answered to satisfaction if you are the chemist in the oil company lab that actually formulates the oil (and that may include the chemist at the base oil company too). Unfortunately the oil companies are very tight with the data they release.

Three parameters that interest me for making oil selections and are very hard to come by are base oil viscosity used, viscosity index imprrover amount, and oil group used.
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