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First number is the "winter" (hence, 10W) viscosity rating, the second is the summer, or high temp rating. The higher the number the thicker the oil (higher viscosity, or resistance to flow). Ideally we might like a 0W-40 or 0W-50, to give best of all worlds. Mobil claim this with their "Euro-formula" 0W-40, and their specs say it has significantly higher viscosity than their 10W-30 at 100C, but also show it has higher viscosity than their 10W-30 at 40C, so this is a little puzzling. Perhaps the viscosity at 0C (the temp at which oils get their "W" rating) does dip below the 10W-30?
My oil philosophy - use the best quality which suits the application and passes my cost/benefit "sniff test". That's Mobil-1 in my view. And use the thinnest oil which will still meet the highest temperature conditions I expect the engine to see. That's 0W-40 for our cars in California and, for my son's 81GL when he's at college in western Canada, 5W-30.
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Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, "my" 94-944 B230FD; plus wifemobile Dodge minivan, hobbycar MGB, and numerous old motorcycles)
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