According to the manual for my Kohler Confidant engine,
"A partial vacuum should be present in crankcase when engine is operating. Pressure in crankcase (normally caused by a clogged or improperly assembled breather) can cause oil to be forced out at oil seals, gaskets, or other available spots.
Crankcase vacuum is best measured with either a water manometer or a vacuum gauge. Complete instructions are provided in kits."
I bought the mower with this engine used. It's a commercial engine, and looked to have been used hard. I fought oil leaks for the first 3 seasons. I finally have them fixed. I think I had bad head gasket(s), which would create crankcase pressure, which would blow oil past the seals. During the last off-season, I replaced the head gaskets, main seal, and resealed the oil pan and valve covers with fresh RTV. For the first time, it doesn't use oil, and the engine and engine bay isn't getting covered with oily dirt. I have a few areas I mow under trees which don't have a lot grass. These areas kick up a lot of dust, and I've been fighting oily dirt buildup since I bought it... until this season.
I recommend getting a vacuum gauge, and measuring crankcase vacuum. I wish I'd done that before doing the "mini-rebuild". I recommend
this one. It's only $25. My crankcase vacuum is good now, but I don't know what it was previously.