My pop's bus appeared to be leaking some oil but we nor my fathers
mechanic could
find the source.   When driving, smoke billowed out of the uper rear
vents.

Most concerning.

On Sunday, there was suddenly a huge puddle under the bus, but the oil
level was fine.  The tranny started behaving very odd with erratic
shifts, and sometimes delays before going into gear.

Pop had it towed to the mechanic as we were now sure it was tranny
fluid.  How might tranny fluid rear the exhaust headers you might ask?

Turns out that there is breather valve or something like that on the top
of the auto tranny.  Apparently as the fluid heats up it also expands
and air is forced out fo the tranny.  If this is blocked (as appears to
have been the case on my dad's van) then with no air able to get out,
fluid travels up the filler tube and pushes past the tranny dipstick
where the tranny fluid the drops onto the cylinder head and from there
onto the exhaust header.

THe mechanic cleaned the valve and all appears fine again.

Hope this helps someone.

...Markus