Did some looking.. not a lot of information available.
As to timing belt tension …
Generally( but not always ) is to have between 1/4 to 1/2 inch deflection depending on the distance at the mid point of the longest distance between pulleys.
7 to 10 inch span between pulleys ~ 1/4 inch deflection.
13 to 16 inch span between pulleys ~ 1/2 inch deflection.
when pressing down with ~3 lbs of force.
Can’t say what this black cap is, could be some type of breather.
Back then car companies were doing various things to engines to try to meet emissions.
Concerning your “chocolate shake” syndrome …
Do you notice this mostly in cold weather and/or using the car on a lot of short (around town trips)
If so, your dealing with condensation.
Your not driving long enough to allow the oil to ” dry out ”