Why the 30 at spec for lawn mowers?

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Straight 30 can handle some serious abuse.

We had an International 806 diesel go to 15,000 hours with no overhaul

Straight 30 all its life
 
Yup, as others have said - no VII to shear down. All oil, all lube
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No plastics to gum up a hot-spotted air cooled motor
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Because a 30wt oil works great. It worked great in my 22 year old Honda walk behind mower, an 20 years in my B&S push mower and has worked great in all the rest of my OPE.
 
if you opt for a PAO or Ester 30W it actually meets the 10-30 spec WITHOUT VII's! you say just a mower but they can get very expensive + as noted air cooling especially at low speeds is minimal, unlike an air cooled motorcycle @ 50 MPH!!
 
Originally Posted By: slacktide_bitog
I use Dollar Tree 10w30 that comes in that 16 oz bottle
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Who is the distributor?
 
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...could be even because they are usually splash lubricated....? But my B&S 3.5classic lawnmover still works after a 20+years on whatever in Sae40...
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Lawn mowers are simple beasts with splash lubrication. Back when Briggs and Stratton first started producing lawn mower engines in the 1940s or so, 30 weight oil was the norm. I have a few antique engines from the 1940s-1960s and many of them specify "SAE 30 in summer, SAE 20 below 32 degrees". I have a 1971 Gravely walk behind that actually says "Mobil SAE 30" on the serial tag.

If you open a Briggs flathead from 3 years ago and one from the 1940s, they aren't much different inside. SAE 30 is also pretty shear stable. More modern small engines like the OHV and OHC Hondas, OHV Kohlers with hydraulic lifters, etc, will specify 10w30 or 5w30 synthetic in the owners manual for improved efficiency.
 
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