oil question

i dont really understand the numbers too well. if you did run a gear oil(80wt) instead of the standard oil, what negative effects would it have? why change oil weights because of the temp outside? as i said, i dont really understand, so any help is appreciated.

Also refer to this chart:


Note that gear lube viscosities and engine oil visosities are not numbered on the same index. An 80wt gear lube would be too light for use as an engine oil for a YZF in most circumstances.

Gear oils lack a number of attributes necessary to a good engine oil, not the least of which is usually that they lack any detergent/dispersant additives. These enable oil to suspend and transport were particles and debris contaminants to the oil filter, where they can be taken out of the system. Transmissions normally have no filter or circulating pumps, and the preference is to allow solids to settle out to the bottom of the sump.

Gear oils are also generally not as resistant to extremes of temperature as engine oils need to be.

OTOH, engine oils usually don't use viscosity index improvers (the additives that give the oil its ability to be a multi-grade) that are tough enough to survive the thrashing dished out in a gear box. This is why it is important to use an oil that is actually formulated for a shared engine/transmission application.

As to changing viscosity to suit the weather, understand that what viscosity essentially is is the "pourability" of the oil; how easily can it be moved from one part of the engine to the other. The volume of oil delivered to the lubricated parts is vitally important, often more so than the pressure at which it is delivered. The ideal is to use the thinnest, or lightest, oil that will provide adequate film strength and protection from wear. Since oil thins with heat, the ambient heat of the environment in which the engine operates contributes to the total temperature of the oil, and when it gets hotter, we compensate by using a thicker grade oil. Conversely, if it gets very cold, we use lighter oils to ensure good flow and delivered volume.

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