Input on 10W-40 vs. 15W-40

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Oct 24, 2002
Specifically, I am considering either the 10W-40 or 15W-40 from Amsoil combined with a dual remote filter on a 1990 BMW 325 that is going to see track use on a roadcourse, as well as street use. For the next few months, it will be a daily driver in MA, then it will primarily be to and from events, and have occasional use during the week.

I figured the 10W-40 would be good in this app, but the 15W-40 appeals to me for a few reasons as well. Primarily, because this car has been run on conventional oil and neglected for 130k, I figured the additive package in the diesel oil might be beneficial. I also like the fact that I believe it has an extra dose of corrosion inhibitors and the car will eventually be idle for days at a time. The only drawback I can see is that the 15W-40 is considerably thicker than the 10W-40 when cold, though it still scores pretty well. Can anyone see any drawbacks to using the 15W-40 under "racing" conditions, as that is really the bigggest issue for me? Has anyone that you know of run this oil under similar conditions, or is it way out of it's element and I'm off my rocker? Any input is greatly appreciated. Thanks.
Not all oil labelled as 15w40 will be thicker than other oils listed as 10w40 though. For instance, we know that the Schaeffer 15w40 can actually be labelled a 10w40 since it qualifies to be classed as a 10w if they wanted it to. But they keep it labelled as 15w40 for marketing reasons since that's a popular weight for farmers.

So do you know for sure that Amsoil's 15w40 is thicker? It might be similar to the Schaeffer oil and could be marketed as a 15w40 when it's really a 10w40 also.

[ November 22, 2002, 05:27 AM: Message edited by: Patman ]
Very true. There are other things to consider but with viscosity ratings that close you're better off falling back to the specs.

Ams HD synth 15w40 Kinematic @100C cSt 15.5, Pour Point -47f
Ams synth blend 15w40 15.1, pour -32
Ams AMO 10w40 14, pour -54
Ams PMO (blend) 10W40 13.7, pour -42
schaeffer 15w40 14-16, pour -41

Kinematic viscosity @100C ranges from ~13-17 for a 40W.

I like your reasoning with the engine age & history. If it were me, I'd completely ignore extended drains and change every race weekend, using analysis to know how things are progressing. If high temperatures are an issue I'd start at a lower viscosity and adapt for wear.


Edit: Gack, I hit return unintentionally... Bob's virgin analysis of Schaeffer 15W40 showed the vis at 14.64.

[ November 22, 2002, 07:27 AM: Message edited by: OneQuartLow ]

These oils use a very similiar additive chemistry will note that the 10w-40 is also CI-4 rated. Both have approx 4300 ppm of calcium and 50 ppm of magnesium. They also have approx the same level of zinc and phosphorus. The 15w-40 does have the extra corrosion inhibitors and the TBN retention seems to be marginally better.

I'd actually go with the 15w-40 for the reasons you have stated. It's about 10% thicker at operating temp, the Noack is marginally lower - 6.7% vs 7.0% and it seems to be more shear stable. It's a good choice for older German cars where you don't want to go with the very expensive S2000, 20w-50.

The difference in cold temp pumping is actually pretty small'd have to go to the 5w-30 or 0w-30 to see a significant improvement in that area.

Thanks for the replies guys. I guess I should preface everything by saying that I am an Amsoil dealer, so I will be using those products. I realize from this forum that I still have a considerable amount to learn, so I wanted to look at this issue with some people with more experience.

With regards to the 15W-40 being thicker, in addition to the numbers posted by onequartlow, I was looking at the kinematic viscosity at 40 degrees, 79.2 vs. 100.2 and the CCS numbers of 2614 vs. 3600. I realize that both numbers are extremely low compared to conventional oil and that the 10W-40 performs extremely well, but I'm anal and looking for the best compromise

Currently, I've got Amsoil's 5W-30 in there, it was what I had on hand. It will be drained shortly after a very short interval and a sample sent in for analysis. I expect that one should prove entertaining here.

Edit: I see TS replied while I was typing. I am definitely leaning towards AME, especially since it's cheaper as well.

[ November 22, 2002, 10:53 AM: Message edited by: mdv ]
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