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Viscosity, HT/HS & turbos
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Posted by Greg Netzner (more from Greg Netzner) on Wed, 30 Apr 2003 09:57:49 Share Post by Email
In Reply to: Re: Dick's right ..., robertt, Tue, 29 Apr 2003 17:58:30
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Hi Robert,

Maybe I should have clarified a little more. I agree with Dick, that Steven's particular application doesn't necessarily warrant worrying too much over which viscosity & filter size he picks. Of course, then I had to go & muddy the waters with my own opinions, right? :-)

People use (or don't use) a particular oil for any number of reasons. Viscosity is the primary gauge for selling & buying oil, with "quality grade" (API/ILSAC or ACEA rating) being a far second. (OK, realistically, the API rating is a far second, while ACEA got lost on the way to the game.) For most of today's Normally Aspirated engines, this is enough. However, when you start talking about turbos, things need to be turned around. And this is why Saab manuals have been recommending ACEA A3 rated oils for years. (Since my post back in January, I've realized GM-LL-A-025 is a "feel good" spec only, & holds no meaning for engine protection.)

From what I've seen discussed here & other places, the increase in viscosity between a 30wt & a 40wt full syn is a non-issue for mileage. More people report mileage INCREASES (or no change) when going to 0W/5W-40 than they do decreases. Higher viscosities provide better hydrodynamic films, thereby decreasing friction more than the thicker oil slows the moving parts down (ie, the internal friction, or drag, cited as the reason to avoid thicker oils). Dick chose his words well, saying, "Doubt if you will be able to notice ANY SIGNIFICANT DIFFERENCE between the oils. The 0W-40 will be a bit thicker at operating temperature, and will THEORETICALLY have more internal friction." [Emphasis mine]

As you pointed out, Mobil recommends their two ACEA A3 oils (0W-40 & 15W-50) for turbo engines. The A3 rating & the newer A5 rating both meet the same criteria for oil stability. The A3 is different in that it specifies a better High Temp/High Shear characteristic for the oil, so it withstands the effects of high-speed turbo bearings much better. Some of the 30wt Amsoil oils meet this HT/HS rating, but none of the Mobil 30wt oils do. Dick uses Amsoil exclusively (I think), & he's stated his preference for these better HT/HS oils in his turbo engines. I'm happy paying a bit less for M1 at my local store, so that's the way I go.

As with most things, it comes down to preference. Thinner oils typically provide better gas mileage, while thicker ones typically provide better engine protection. Synthetics have blurred the dividing lines quite a bit, & legal wrangling has blurred things (like oil classifications) even further. Since running Mobil 1 15W-50 in my N/A C900 for the past winter (down to about 12*F), I really don't see any reason NOT to use THIS oil in a climate such as yours or Steven's. (My gas mileage remained the same as with the 10W-30 Syntec I was using for the previous 40k miles, & cold-starts were never an issue.) Your turbocharged engines would have ultimate protection. But most folks are unwilling to go "that thick" after a lifetime of 30 wts. 0W-40, in my opinion, is the minimum protection our turbos should get.


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