Explanation of cold/hot vis - Saab NG900 & OG9-3 Bulletin Board - Saabnet.com
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Explanation of cold/hot vis
Posted by Eiron [Email] (more from Eiron) on Tue, 15 Nov 2005 09:50:08
In Reply to: Are you sure, Finch, Mon, 14 Nov 2005 20:52:58
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Let's take 10W-30 as an example. The rating only means that the oil flows like a frozen 10 wt when it's really cold (minus 20°F) instead of a frozen 30 wt (which would be thicker than you'd like at minus 20°F) & then behaves like any 30 wt would when it's hot (210°F & hotter). The cold rating (xW- end of things) should be viewed as being completely separate from the hot rating (the 30, 40 or 50 wt part) & is simply a labeling convenience to help you pick an effective oil for your start-up climate. The hot rating is where you'll get all your operating temperature protection, & should be chosen based on the engine's design & protection needs. Separate them in your mind!
Done that? OK, good! Now consider this: ANY motor oil will be thicker below 100°F than ANY OTHER motor oil will be above 210°F!! What does THAT really mean? It means the "thinnest" 0W-20 syn will be thicker for all of your cold starts than the "thickest" 20W-50 syn will be once it's flowing & the engine's warmed up to operating temperature!
5W-30 & 10W-30 are both "thinner" & provide less turbo protection than M1 0W-40!! The best turbo-protecting synthetic oil you can use is 15W-50. (As long as it doesn't regularly get below, say, +20°F, you're fine.) Colder than that (regular drops down to 0°F) & 5W-40 would cold-flow better but still offer great turbo protection. Colder still (maybe routine minus 20°F mornings) & 0W-40 would be your first choice.
For Saab turbos, you really want to stay with an ACEA A3 rated oil. This is the most durable High Temperature/High Shear (HT/HS) classification, & ensures the oil will survive the harsh environment of your turbo. Operating at well over 400°F & 100k rpm, turbos are the definition of HT/HS! Any of these three Mobil 1 oils (15W-50, 5W-40 or 0W-40, or comparable oils from Amsoil) are your best bets.
posted by 146.82.1...
Posts in this Thread:
- 99 9-3,close to 70k,what grade of synth oil to use now?, AlexA, Mon, 14 Nov 2005 18:37:17
- Why do you want to change?, Eiron, Tue, 15 Nov 2005 09:55:37
- Re: 99 9-3,close to 70k,what grade of synth oil to use, Dean, Mon, 14 Nov 2005 20:26:37
- Re: 99 9-3,close to 70k,what grade of synth oil to use now?, Jeremy R., Mon, 14 Nov 2005 20:15:33
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