Harry B.,
Sorry to take so long in replying –
I don’t want to fuel the Great Oil Controversy again (reminds me of political discussions and I don’t want to go there). My opinions on lubricating oil for air-cooled engine applications were formed when, working for Tecumseh products as a mechanical engineer, I was in charge of evaluating various oils for extended oil change intervals. Long story short, the multi viscosity, full synthetic oils far exceeded both straight-weight and multi-weight mineral oils in all respects. Due to the high operating temperatures encountered in these small engines, oil temps can actually exceed 300 deg F under extreme conditions and this quickly causes oxidation and a viscosity increase due to boiling off of the light fractions. The synthetics react much slower to high temperatures and remain serviceable by a factor of approximately 3 times longer than mineral oil equivalents. The lower rate of oxidation keeps the synthetic oil from darkening as rapidly and with stable viscosity, allows oil change intervals to be safely extended.
Lubrication chemistry has evolved greatly since the “low-ash” oil recommendation for Kohler engines, made over 40 years ago. Modern synthetic oils have the stable, chemically formulated base stock combined with additives that both lubricate and protect engine internals much better than the equivalent mineral oils. It is possible to run a 10W-30 full synthetic all year round in your Kohler and you’ll be surprised at how clean the oil appears, even after 50 hours of use. Oil consumption will not be increased as compared to straight-weight oil either. Sticking exhaust valves are a thing of the past, and combustion chamber deposits are greatly reduced. Any reputable brand of full synthetic, such as Mobil 1 or Amsoil, is a good choice.
These are just my opinions based on actual endurance testing of small engines at (what was then) a major engine producer. I’ve run Mobil 1 in my K301 year-round now for the past 12 years and have never experienced any problems. It gets 1 oil change per year, always starts on the coldest of days, and hardly ever requires topping off of the oil level. I would never try and convince others that this is the only way to lubricate their engines – I’m just sharing my experience. And I appreciate your asking.
Dave