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Just to add two more pennies to the pile, the first number is the base weight of the oil. The second number refer to the viscosity enhancers. The base oil follows the standard tables for "thinning out" with temperature. The enhancers make the oil act like it is thicker when it gets hot. The enhancers are supposed to follow the SAE guidelines but there are differences between manufacturers and types of oil. Multi viscosity oils allow us to drive over a wide range of temperatures without changing oil.
That being said, the engine build is set up with internal tolerances that dictate the weight of oil for different temperatures. A loose race engine will have large tolerances and will "wind" faster and have less internal friction; but will need a higher base weight oil (often a strait 50w).
The other consideration is the other additives in the oil. Race oils have different properties than street oils and are designed for short term use. Do you change oil after each track session? Do you have a track built engine?
Ok, so I got wound up and spent four cents.
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