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Re: oil weight question (archive)

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Posted by Bob ///M3 on June 21, 2000 at 07:29:35:

In Reply to: oil weight question posted by Bimmerfan on June 20, 2000 at 23:26:16:

The multi-viscosity oil will get thinner as it gets hotter. The first number is rated at a lower temperature and the upper number is rated at 100 C (212 F). A 30 weight oil at 212 F is thinner than a 0W at 90 F!

Think of it this way; when the oil is cool you have the weight of the lower number and when the engine is "at" temperature you have the weight of the upper number. With this in mind, I want the lowest possible first number to reduce dry start-up damage at the bearings and cam lobes and the "proper" upper number as recommended by the manufacturer in the coolest climate conditions (since BMW based this recommendation on the use of petroleum oil) which is 30 weight.

A 30, 40 or 50 weight oil when at normal crankcase temperature, say 100 degrees C (212 degrees F), is thinner than a 0W** oil is when cool, at say 80 degrees F! It's like maple syrup; when cool or cold its very thick and slugs out of the bottle but when hot it pours out like water! But, if you put that same hot watery syrup in a frying pan over a fire and get it too hot it will thicken-up like tar. That of course would be too hot for the syrup! With the synthetic oils, we don't have to worry about the oil becoming too hot as all of the synthetics are better than any of the petroleum based oils at "flash point" or "fire point" ratings.

Why would anyone want a thicker oil at (dry) start-up than they would have at normal operating temperature? The answer is we don't! The best we can do is use the lowest first number available which in most cases is 5W-** however some makers offer a 0W-** to reduce the cold (dry) start-up, metal-to-metal contact, that causes a hundred times more damage than normal engine operation when the engine and the engine oil is at normal operating temperature.

Bob ///M3


: so if 30 weight oil is thinner than 50...
: and if 0w is thinner at cold startup than 10w...
: I'm inclined to believe due to above data that a lower number is less viscous in general.
: SO!
: does that mean that as the oil gets hotter (engine warmer) the oil should approach the second value... so the oil gets thicker as it gets hotter?
: Maybe I've got this all wrong.

: I just had a mental chemistry paradox...




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