[Y-Blockhead]

Originally Posted by 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.

Thanks Werner. I don't think either Penrite or Lucas 85W-140 are synthetics (altho Lucas does make a 75W-140 Synthetic Gear Oil). Those are the only gear oils that have been in this transmission. I don't believe the oil was contaminated. We'll see how the Lucas works... again.