With some planes as I recall it was a problem of oil starvation.
the Allison engines /p-38/p-40/early 51's, could only run inverted for about 30 seconds if i remember right, it had a dry sump oil system but no way to pick enough oil up from the valve covers to sustain pressure,
the main pickup was in the pan, there was also a line from the back of each valve cover about 1 inch I.D. , nose down inverted zero or negative G was a big no no!
the lines in the back of the valve covers were there to drain off access oil during nose up flight but could also help feed the pump for a short time in inverted flight as long as you kept the oil at the back end of the motor!