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Hi jkrowas,
Keep in mind that the "xW" end of any oil's rating is always going to be a sub-zero specification. (The actual sub-zero rating changes with each "xW" tier.) This means that your oil's actual viscosity in normal operating conditions (I don't know what typical winter startup temps are for Boston) will not be a hard-&-fast value equivalent to the "xW" number on the label. Let me try to explain it this way:
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 (let's say 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 will be thicker for all of your cold starts than the "thickest" 15W-50 will be once it's flowing & the engine's warmed up to operating temperature!
This means 5W-30 & 10W-30 are both "thinner" at typical startup temps & 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 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.
I live in Northern Colorado, so a single year's operating temps will see 0°F cold starts & 100°F stop-n-go traffic. And I'm not covering how the hot end works WRT conventional vs syn oil.
I'm adding a link to Saab's July, 2000 synthetic oil TSB which states that ALL Saab vehicles have been factory re-spec'd to run fully synthetic oil. Any "real" full syn dual-rated oil (approved for use in both gasoline & diesel engines) will provide much better turbo protection than any gasoline-only rated oil. The dual-rated oils are formulated with much more robust additive packages.
Of course, some of the changes in oil viscosity ratings are North American specific in order to meet CAFE requirements. But that's an entirely different discussion....
posted by 57.80....
, Wed, 5 Nov 2008 15:44:14
, Tue, 4 Nov 2008 16:02:37
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