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There’s a long winded technical answer, but I’ll try to be brief. Lubrication is dependant on flow. Viscosity, as it's measured in motor oil, is a measure of a fluid's resistance to flow, or it's "thickness". That "thickness" is quantified in centiStokes (cSt - Stokes Law for Friction). Motor oil thickness is temperature dependant. The normal operating temperature for motor oil is 100°C. Using the Mobil1 product data sheets available at the Exxon Mobil website, you can see the specs for Mobil1 0W-40. At 100°C, Mobil1 0W-40 is 14 cSt. I’m going to *guess* MB powertrain engineers built tolerances around that number. If the motor oil in every M113 was always in that range, there would be no wear, and they would outlive most of us. Unfortunately, when the engine is off, oil cools. At 40°C, Mobil1 0W-40 is 78.3 cSt. That’s a little thick for good lubrication, which is why people are told to stay off the gas with a cold engine. I wouldn’t use a motor oil that doesn’t meet MB spec because *I think* there’s a good chance that it’s either too thick when cold, or too thin when hot. Mobil1 5W-40 is an example of what I’m talking about. Mobil1 5W-40 is over 100 cSt at 40°C.
But hey, owners should use whatever they feel most comfortable with.
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