If your mowing temperatures are above 90 F, I'd go with Kaw. 20W50. If your mowing is below 90 F,
I'd use Kaw. 10W40 or CK4 15W40. In my ZTR I use Mobil 1 15W50.
I had the same problem on a Honda powered pressure washer. Plug looked OK, (about 50 hours run time) but changed it anyway.
Old plug NGK BPR5ES replaced with same. Engine ran perfect.
I have a Snapper Pro S200 (made by Ferris) with a Kaw FX-850. Been using Mobil 15W50 and Napa 1394.
In the summer heat these air cooled engines run oil temperatures above 260F. Kawasaki oil chart recommends
20W50 for high ambient temps.
Last saw I bought is a Milwaukee 18 volt with a 12 AH battery. Cuts a lot of wood on one charge. No gas mix to worry about. Never
hard to start. Very powerful. With 2 batteries you could work all day. Check it out on you tube.
I have a snapper pro with a Kawasaki FX 850. Mobil 1 15W50 is perfect. These air cool engines run a lot hotter than water cooled engines. Under load
my oil temp. is 250F plus. Also use 15W50 in the hydo drive.
Use a clean 5 Gallon gas can, put in 1/2 gal of TCW3 and fill with diesel. Pour this into storage tank. Repeat with remaining TCW3.
You might want to do another 5 gal of diesel just to agitate or if you have an electric pump use nozzle in tank fill hole to agitate.
I would have the fuel transfer pump check first. These have a
history of leaking into sump when seals go from using ULSD fuel. I think this is an ISL engine?