That yellow stuff is water mixed with oil, it has no place in the crankcase. Likely plugged PCV system. It looks like lots of short trips in cold weather or you have a bad thermostat that allows the water jacket temp to be too low. Likely it has a bypass closing thermostat and the bypass has to close when cold or the engine does not warm up fast enough or stay low in cold weather no matter the loading.
Definitely use a good synthetic with detergents and change more often - like 3000 miles. The marvel mystery oil added to a hot engine then driven for 30 miles (hot coolant) then drain may help. You can scrape some of the deposits into a number of containers and mix some oils and treatments in with each to test where you can see them. The worst case is big slugs of sludge gets to the oil suction bell, so don't do anything drastic.
Definitely use a good synthetic with detergents and change more often - like 3000 miles. The marvel mystery oil added to a hot engine then driven for 30 miles (hot coolant) then drain may help. You can scrape some of the deposits into a number of containers and mix some oils and treatments in with each to test where you can see them. The worst case is big slugs of sludge gets to the oil suction bell, so don't do anything drastic.