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Subject: Re: gear lube / motor oil viscocity difference?
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Date: 05/30/02 at 11:23 AM
Posted by: Rich L
In Reply to: gear lube / motor oil viscocity difference? posted by Mike Painter on 05/14/02 at 3:54 PM:
would like to know the difference between 75wt-90 wt.and 50 wt.motor oil.
The only real difference between any lubricant that claimed to be "the same" but with different weights is the amount if viscosity index improvers. For example, a gear oil that is labeled as 75W-90 will have an ability to stay within the most functionally appropriate viscosity for a narrow range of temperatures. The W next to the 75 means "winter" and that number indicates lowness of viscosity at winter-like temps. The 90 is the viscosity rating for when temps go high. On the other side of the coin, look at other gear oils (or motor oils) that have a wider range between the stated numbers; 85W-140. That represents a much wider temperature range of applications. This means that the oil will hold its proper viscosity (won't thin out) at higher temps as would the 75W-90. However, the only drawback is that a higher visocity index means that viscosity index improvers have to fill out some space for every given ounce of lube...hence less "room" for the other additives that go into oil such as the oil itself. (Just in the way that strawberry lemonade can't be all strawberries or it wouldn't be lemonade!) So oils have to do their job without become so much of another chemical that it's original function is lost. Perhaps this is where some people swear by straight sae30wt. motor oil. It would have the least or none of those viscosity inedex improvers. However, todays hi-perf engines and machinery need lubes to be able to be behaviorally stable at a range of temps so one should take advantage of the technology but within the appropriate range of temps. Manufacturers will have their recommendations, taking this into account.
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