If a 660 was ran on Ultra @ 32to1 and tune rich and the air filter kept clean the oil will perform well and actually keep the piston/exhaust clean and the piston top will look alot like Amsoil Saber does with carbon on the crown. Problem is that the average operators cant tune, wont clean the filter, runs 50to1, runs a dull chain, etc. Therein lies the issues with most oils that WILL cause issues before other oils will. Different oils perform differently and as your test are showing that when one is pushed over the edge in undeseriable conditions , some do a better job than others. Most of us here that can tune, sharpen a chain, keep a clean filter, mix fuel correctly, can use most oils without issues IMO