somehow you have got water in the crankcase. this can happen if tractor is parked outside in rain.the raqin can enter thru upright exhaust and seep into oil pan. dump the oil and put new oil and oil filters in. the sludge on oil filler cap is condensation of oil and water created when engine is at running temp. from the water in the oil pan. changing oil will remedy this.as you said no running problems and clear exhaust, this is not a head gasket problem a hint for you,is to place and old can upside down over the exhaust when parked out in the weather.this will keep moisture and rain out of the engine,not to mention the blck crap that will shoot out when you start engine
It's diarrhea that is frothy and watery.
Pumice is a frothy, bubbly, light volcanic rock.
"Frothy" means a slight foam has appeared at the top of the liquid you are whipping or beating.
rapids
It's meant to mean that they have rabies. Some dogs just get frothy mouths when the run around a lot.
Frothy eloquence refers to light and entertaining statements that offer little substance.
Pumice
foam
A frothy texture.
soda
pumice
The wake