BTW: 5W-30's not exactly "thinner" than 20W-50... (archive)

The SAE numbers refer to the viscosity at a given temperature, but 5W-30 and 20W-50 have almost the same viscosity at normal running temperatures. At low temps, i.e. during cold starts, the 5W is thinner than the 20W. This is GOOD, because it pumps through the engine better, provides better lubrication for cold rings scraping against cold cylinder walls, but doesn't put as high a current demand on the starting / charging system. There's an oil pressure relief valve that bypasses the engine's oil galleys above a certain pressure, to keep cold hard oil from blowing out seals etc. So thicker oil doesn't do anything good in a cold engine.

At normal running temps in an engine in good condition, you won't see a significant increase in oil pressure going from -30 to -50 multigrade. Only at higher running temps under higher-than-normal loads will the -50 really be thicker than the -30, as viscosity starts dropping off at a lower temp for the lower number oil. But increasing oil pressure beyond what's needed is BAD.

You don't need high oil pressure - you only need enough to keep a thin film of oil in the interfaces between crank journal and bearing surface, ring and cylinder wall, etc. Thicker oil robs horsepower (it takes HP to push the stuff through the oil pump and passages), increases engine temp (since oil is both a cooling medium and a load), and wastes fuel.

BMW doesn't want to replace a bunch of engines under warranty, so their engineers are telling you what's optimal for oil grade / viscosity / range, oil pressure, etc. You maximize power and engine protection by using the "thinnest" (i.e. lowest SAE number) oil that BMW says is safe in your engine.

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