The thing that is ironic regarding this thrashing of straight 30w oil for OPE is that the 2 largest manufacturers of commercial OPE engines in the U.S. (Kawasaki and Kohler) still recommend 30w and/or 10w30 as the preferred oils for their engines.
While they might not have dedicated lubrication engineers on their staff, its also hard to believe that they have their heads in the sand. They sell many thousands of engines annually. The pros over at lawnsite.com talk of using 30w/10w30 as much as using 5w40 or 15w40, so these oils are not as obsolete as lead to believe here.
535678, regarding shearing, air cooled engines shear oil more than liquid cooled. Whether its mechanical design or higher temperatures, I am not sure (probably both).
Straight weight oils, by design, shear LESS than multi viscosity oil. Comparing shearing of a quality dino HDEO 30w to a synthetic 10w30?....does anyone have data about this? However, it is this less shearing feature that some consider as a strength of straight over multi viscosity for air cooled engines.
My personal preference is to use HDEOs in air cooled OPE engines, with NO proof that automotive SN oils are not up to the task. Just my bias, supported by Kohler's marketing of HDEO OPE oils (and other reasons):
http://www.kohlerengines.com/accessories/kohlerOil.htm
-Rotella has inexpensive HDEO oils in 30w and 10w30. The Kohler HDE0 oils above are overpriced at $5-$6/qt.
-Amsoil, Exmark, and others market synthetic dual 30w-10w30 oils for OPE for $7-$8/quart.
In the end it doesn't matter much. Its a personal choice based on which Kool Aid you drink.
Take care.