This is the way it was explained to me. No expert knowledge follows.
On 4-stroke engines with no oil filter you want to run the straight 30
weight, low detergent oil since it is formulated with very few
surfactants (I think that is the right word for oil molecules that pick
up other molecules and carry them with it) so the bad particles will
tend migrate to the bottom of the sump and not be circulated with the
oil.
Multi-vis automotive oil has lots of surfactants to carry bad particles
to the filter where they are strained out and removed from the engine by
disposing the filter.
So, the rule of thumb I was given for small engines is:
No disposable filter - 30 weight, low detergent only
Disposable filter - just about any automotive engine oil will work
Of course small engines are very forgiving of oil related abuse. The
statistical difference in engine life between 30 weight and high quality
automotive synthetic would probably be lost in the noise.
Jon Rush
Douglas Shook wrote:
>
> Hi,
>
> I know all the B&S and Tecumseh manuals state to use SAE
> 30 weight oil, and that is what I always have used, until
> my last oil changes, on my 3.5 hp Tecumseh mower and 3.5
> hp Tecumseh pressure washer.
>
> Since they only take a pint each, and I only usually
> change their oil once a year, this time around I put in a
> synthetic blend 20w50 CF/SJ rated oil.
>
> My thinking here is that even for my air cooled Harley,
> they recommend CF/CH 20w50 (diesel oil spec). They only
> recommend straight weight oil if the ambient temperature
> does not drop below 75 degrees.
>
> Unless I am missing something, I believe that engine oil
> has progressed a long way since running straight 30, and
> that even these little flat heads could benefit from a
> multi-viscosity synthetic blend.
>
> The 20w50 borders the 30 wt spec nicely (a little thinner
> for easier starting and better lubrication when cold, and
> a little more protection when hot).
>
> Maybe a 20w40 would be a closer match, but the question I
> am asking is, I suppose, is why have the small flat head
> engines retained an oil spec that, for all other internal
> combustion engines, was abandoned years ago? Maybe they
> believe it is still "adequate," but would these small
> engines not benefit from using more modern oils?
>
> Is there some reason to avoid the newer, more modern oils
> in these little flatheads. I am just curious as to why
> they retain this spec.
>
> thanks,
>
> doug
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