When motor oil ads on TV mention viscosity breakdown, you can imagine they're talking about your oil wearing out, but what exactly is going on in there? Oil is a viscous liquid that thins as it heats up in temperature. Because of this phenomenon, the oil industry measures oil in sub-zero conditions, at 100 degrees C, and 150 degrees C. With multi-weight oils, like 5W-20 (thinner) or 10W-30 (thicker), the W stands for winter.

Most conventional multi-grade oils are infused with Viscosity Index Improver, an additive that helps it resist thinning as it heats up, and keeps the viscosity more consistent as the temperature rises. Picture the Viscosity Index Improver molecules as spaghetti noodles. When it's cold out, the noodles curl up, and don't alter the viscosity of the oil, allowing it to flow as quickly as possible through the nooks and crannies of the engine. As it heats up, the noodles straighten out in a way that keeps the viscosity consistent. Viscosity breakdown happens as these molecular index improvers,

(i.e. noodles) travel through the motor, and get spliced up by gears in the oil pump and other mechanical hardware. When enough of them get cut, the oil's ability to maintain viscosity changes and can render your oil less viscous, or the equivalent of taking a 5W-30 down to a 10W-20. It's also important to note that synthetic multi-grade oils have naturally more consistent viscosity, and use very little, if any viscosity index improvers, and straight weight oils also don't use the index improvers and therefore don't suffer from "breakdown."