I always think its best to follow the manufacturer spec specially for viscosity and specially the last set of numbers on an oil container.

the heavy 40 hot viscosity oil is twice as thick as the 20 one, This might show as high pressure but low volume being transferred (bypass piston closes earlier - saps more hp - saps more mpg too) - we need a high volume of oil to get to the engine parts to cool them down sufficiently. btw whats the problem with regular dino 5w20 oil which is available nearly everywhere. Its meant for the engine and its proven to keep it in good shape.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)