The threads are worn and the vibration of the engine causes the oil cap to loosen. Replace the oil cap or try a little teflon tape on the male thread as a temporary fix.
if the float/needle assembly do not seat properly. the gas will slowly leak past the needle and into the cylinder-then past the rings into the oil. you probably need a carburetor rebuild.
I'm guessing that it's engine oil you mean. It can come from a bad oil cooler if it's got one . Could also come from a bad cylinder head gasket, leaking seals on wet type cylinder sleeves, warped...
More info is needed. Sears uses many different engines. We will need to know which engine is in the mower. There should be a tag or sticker on the top of the engine with the brand and HP.