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If we're talking about dino oils, then yes, the oils that have the lower xW viscosities are also going to have thinner hot viscosities. Also, don't forget that SAE vis weights are only ranges, so one company's 30wt can be quite a bit thinner than another's.
Think of it this way:
All oils are thicker cold (below 100°F) than they are hot (above 200°F). Syn oils don't thin out as fast or as much as dino oils do, so they can be made thinner than dino when they're cold & still end up thicker than dino when they're hot. A dino oil doesn't have the ability to stay thick when it gets hot, so it has to be made thicker to start.
I've read saabspokane's recommendations & generally agree with them. But it's NOT having a thick xW rating that protects you car's engine. The xW end only tells you how well the oil will pump when it's really cold outside (ie, sub-zero). If it never gets below +20°F, then use M1 15W-50 in your turbo all year long.
Where the engine & turbo protection come into play is when things get warmed up. THAT's when you want the most durable film of oil you can get, 'cuz that's your primary & most effective means of protection. At 400°+F & 100k+rpm, M1 15W-50 is best, followed by 5W-40 & then 0W-40. All three of these oils have higher HT/HS ratings & provide stronger oil films. Once the film's "cushion" is gone, you're relying on the anti-wear additives to keep metal from metal. That's one reason why Saab's oils are diesel-style formulations. Another is that turbos generate a fair amount of crud, & diesel oils are designed to handle tons of it. M1 15W-50, 5W-40 & 0W-40 are all dual rated for both gasoline & diesel engines.
As long as you're sticking with a premium (Group IV/PAO) full syn (like Mobil 1 or Amsoil), then you have little to worry about in terms of engine protection & oil durability. Stick with something that's ACEA A3 rated (usually a 40wt or 50wt) & has the xW rating necessary for good cold flow in your area. The better durability & additive packages of dual rated syn oils provide the best wear protection & engine cleanliness.
posted by 146.82.1...
Too much information, driving me insane
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