Usually it's either 1 of 2 things-either the fluid comes up & out slowly & coats everything with oil to later drain off & make a mess after the truck is shut off or the air gets trapped inside the circuit to later "burp out" once the circulation of the oil has stopped

Had a Sr. Technician come by and look it over today, and he said something similar, the lift is 41 yrs old, likely the o-rings in the trans as well. He thought a degraded trans pump o-ring could cause air to be pulled into the fluid. The fluid did look a bit opaque in the bottom of the fill tube, but didn't contain visible bubbles. Fluid on the dipstick and floor looked clear, which supports temporary air in fluid. Tech also mentioned that carbon seals wear out leading to lag in response of transmission of power when the lift is put into gear. Later in the day today, an employee complained of just such a lag, and lacking power which prevented the lift from being able to pull its smaller back tires over a lip at the edge of of the floor slab at a warehouse entrance. Why would foamy trans fluid cause it to "burp" after it's turned off and parked?
Thanks so much for the help!

Sounds like an automatic (clutch/brake)- if this is the case my best guess is the trans pump is s*cking air creating a foaming situation which in turn raises the fluid level until the air escapes. Check to see if the trans oil is foamy while it's running

Serial # 39y01673. Not sure of the type of trans, has 3 pedals-a throttle, clutch/brake, and brake only. Gear selector on steering column.

if my memory serves me, there were a few transmission options in this model range
is this a hydostatic trans, or wet clutch manual, or torque converter??
does it change directions with a foot pedal or linkage on the steer column?
serial number?