Jul 18, 2009 · Even back in 1979, Case specified SAE 30 or 20w40 for temps above 30degF for the Kohler K241 10hp in my little Case 220. This would go for any K-series. I've got Rotella-T 15w40 in mine. For Kentucky summer heat, I'd use a 40wt HDEO in that "K" for sure. I wouldn't go $$ full synthetic in an old K. They burn kind of rich and dirty. Oct 02, 2020 · An engine running lean is the reverse of one running rich. In this instance, there is more air than fuel being supplied to the combustion chambers. Just like when the engine is running rich, you will need to check your fuel injectors for any clogging. This can lead to less fuel being supplied.