Ok with the johnsons you have the linkage goes under the motor to
the lower unit.

Now follow the linkage from the throddle to the back of the motor with
the cover off. You SHOULD see the levers (on the left side) moving
back and forth. One should be for the engine to rev and the
other lever would to engage the drive.

As for the oil being creamy in the lower unit, that can be a problem.
If it looks like coffee moca, there can be water in the oil. Thats a
problem. (bad seal) I would at the very least change the lower unit
fluid and
adjust the linkage. Its not exactly uncommon for older boats
to have linkage that was stretched out beyond its useful life.

If you want to play with the linkage while the motor is running
(out of the water) I would first and foremost, remove the prop.
(dont need to be standing too close and get caught up by it)
Put the muffs on and fire up the water. You should be able to
disconnect the linkage from the motor and shift it forwards and
reverse. If that doesnt work, you might have a issue with the lower
unit. The other think that can be a problem, is that the last guy
who did the lower unit may have tightened or loosened the
linkage line from the lower unit. Do that a bit much and it
can have issues like you mentioned too.

If there is no real history on the motor, since you are the new
owner I would also look into changing the fuel filter, thermostats,
and water pump impeller. Those are maintence its which may
have been neglected by the previous owner.

Tom