Yellow junk in oil top
 

That is called lung butter in the rotary world. Its a result of condensation collecting over time and depositing gunk there at the top of the oil fill tube. If you are completely stock with the full rats nest, you will need to make sure all the evaporation emissions stuff is working. If you have stripped out the rats nest you need to hook a PCV valve to the nipple on the oil tube and have it attached to a light vacuum source like the air cleaner. That way the moist air can get evacuated from the block when the car is running. Personally, I run my tank vent line into the nipple on the center iron near the base of the oil fill tube and then have the pcv valve on the oil tube nipple and it goes to the bottom of my air cleaner.

plenty of info on this issue. just do a search for lung butter.