[Werner]

Blockhead,

there is an additional reason: If synthetic oil is mixed with mineral oil, it can cause individual components to be incompatible and cause the oil to decompose.
If I understood correctly, was the foam still present within a week? Then that's a sure sign that the oil is contaminated. Since it then loses its lubricity considerably.
It makes sense to carry out a complete gear oil change and perhaps to attach a dipstick to be able to control the formation of foam.