>dirtbeard wrote:
>>... and the manual from Tecumseh explicitly says "do not use
>>muti-weight oils due to increased engine wear and oil consumption."
>
> If it's in the Tecumseh manual, isn't that the final word on the topic?
By no means.
>
> The designers know what makes the engine run best. This
>information goes into the manual. Maybe we'd be better off with a 90
>page wear study on why multigrade is bad, but for whatever reason
>they don't explain, multigrade is not recommended, and they aren't
>just saying this to waste ink.
The people who write the manual (who are very rarely the designers)
also have to take in to account the user of the product. The average
user of a lawn mower isn't too mechanically inclined. Ask your
neighbors when the last time they changed (or even checked!) the oil
in their lawn mower, and I expect that you'll get a blank look. One
really good reason to recommend a straight weight oil is that they
don't have viscosity improvers. The straight oils maintain
viscometric properties lots longer than petroleum based
multi-weights. They'll maintain them well past the point that the oil
is full of debris, ash, and other assorted nasties. That's important
because it keeps oil consumption down. Small air-colled engines have
very small oil capacities -- a pint in the example engines, so even
small levels of oil consumption can be fatal, given that it may go
hundreds of hours without having the level checked.
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