Oil Breakdown

T56MaxTorq said

The simple answer is when you have a weight like 5w50 like you do, the additives that make that large viscosity spread take up space in the oil that would otherwise be lubricant.

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This needs a bit of added info: It's not as true for the synthetics.... they keep their viscosity more stable over temp changes than regular oils and need lower levels of VI's. So running a 5W50 synthetic would be like running a 10W30 dino oil as far as the levels of VI's. (As a SWAG....)