I'm 99.9% sure that the KT series is the post Cast Iron series, maybe 2 series later. I also think the KT17 has a factory defect if it's not the later model updated engine with full pressure lube, the issue was no lube from any slight incline while mowing... this is a known issue, and I may have the wrong thing stuck in my head, however do a google search the KT17 series II is the updated version.
As long as your engine is not equipped with hydraulic type lifter or whatever it is that Kohler deemed needed multi viscosity (5w-30)
All that said, I run either Rotella or Delo HD30 in my Kohler Cast Iron series engines, and also in my B&S Intek 24 hp, I have been inside many of the Kohler Cast Iron series and the HDEO keeps the engines spotless if anything near a routine maintenance schedule is used, I change my oil once a year, and at times I even neglect changing the filter on the B&S.
I'm not sure a filter is even needed on these engines, the Kohler Cast Iron series may have been the best small/medium sized air cooled engine ever built. I have had engines that are original 76 model for instance when I purchased was very dirty, varnished up, sludge etc, however a seafoam treatment ( I guess to thin the oil) and a couple runs of HDEO Delo or Rotella and the insides are super clean with only a very small collection of thick black oil which is not sludge at the center of pan.
