High viscosity vs low viscosity oil on track

SlickShoes · Jun 13, 2003

Interesting thread over on NASIOC on whether its better to run higher viscosity oil while on the track, or not.

http://forums.nasioc.com/forums/showthread.php?s=&threadid=375446

For you people to lazy to click through to the thread...

For whatever reason, I long believed that a heavier oil was necessary when racing your car. I took my 02 WRX to a track day at New Hampshire International Speedway in August of '02, and filled the sump with Amsoil Series 2000 20W-50. Since I like facts and loathe baseless opinions, I had the oil analyzed by a lab afterwards. I was displeased with the results. Curious if the problem was oil that was too thick, the next track day I did, May '03 at Lime Rock Park, I ran Amsoil 10W-40. Once again, the results were analyzed. This time, they were much more acceptable. I will post the results from both below.

<font class="small">Code:</font><hr /><pre>--------------------------------------------------------------------------------
date sampled 9/17/02 6/4/03
miles on unit 15931 33841
miles on sample 2940 3114
oil used S2K 20W-50 AMO 10W-40
visc. @100 16.8 13.83
Oxidation 15.6 8
Nitration 12.9 11
TBN 12.5 9.84
Iron 17 7
Chromium 0 1
Lead 93 20
Copper 6 11
Tin 5 0
Aluminum 9 5
Silicon 9 14
Boron 32 1
Sodium 3 4
Magnesium 383 23
Calcium 3168 3412
Barium 0 2
Phosphorus 1325 1156
Zinc 1618 1185
--------------------------------------------------------------------------------
</pre><hr />
Lead, and maybe tin, represent bearing wear. These numbers are both drastically higher with the 20W-50. 20 ppm lead could even be considered normal street wear on some cars (though not Subaru's in my opinion). 93 is definitely through the roof. Iron and aluminum are both higher, though not drastically. These represent piston on cylinder wall wear.
Both oils, from what can be discerned from these results, are still intact and still capable of many thousands of miles more in a street car. This is based on their reserve TBN and their viscocities, both of which are still "in-grade". The TBN with the 10W-40 is probably more like 11.8, different test method from the 20W-50.

So, is this conclusive proof that heavier oils are not better for racing? No, of course not. One test is just food for thought, it would take scores of similar tests to begin to prove anything. But I thought I would contribute these results so that some objectivity can enter the old "which oil is better" debate.

Some people then go on to mention that its possible the thicker oil is not penetrating the smaller areas as well, and therefore there is actually less protection. Hmm..

Anyone here have any opinions?

belacyrf · Jun 13, 2003

Chris... I did a tad bit of research on this same thing a while ago. Just seeing what the masses in the newsgroups said about oil weight and racing. One thing that I read was that the heavier oils have more additives which increase the viscosity. But they mentioned that with those extra additives, it would be more prone to breakdown, which then leaves you with a lower weight oil with contaminents.

Now I don't remember if this was in regard to Synthetics or not, it might have been normal oil. But it was somethign that caught my attention. And IMO.. 10-40 is still kind of heavy, I mean most 4 cylinders are running 30 weight aren't they? I dunno... I'll go back to sitting on my brain

SVT121S · Jun 13, 2003

Very intrusting, it gives much to think about in regards to selecting what weight oil to use. hhmmm

rheacer · Jun 15, 2003

As I've said before, I'm no engineer, just a weekend racer for 25 years, but oil weight choice depends largely on how engine is built and how it is used. A full race engine is going to be built with greater tolerences, as you plan on engine getting very hot and parts expanding. This would be a great place for heavier weight oil. A modern street engine is built with much closer tolerences, as clacking on warmup would not be, um, undesirable. A lighter weight oil would jump into the smaller opening. Ok we try 5-20W in our street car and it works great, low resistence, great mileage, pretty good protection. I tried running 10-30W in a race engine, to get all the HP I could. Unfortunely, as race temperatures climbed, I lost pressure because oil was just too thin. Cooled off a lap and everything went back to normal. Race oil in a street car? Waste of money. Same with street weight oil in a race engine. IMHO

High viscosity vs low viscosity oil on track

SlickShoes

belacyrf

SVT121S

rheacer

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