The oil level is low.

Add oil in order to acquire the proper oil level. Refer to Transmission Fluid Check .

Air is leaking at the intake tube due to a damaged O-ring.

Replace the intake tube seal and filter. Refer to Allison service manuals.

The oil pan filter is clogged.

Replace the oil pan filter. Refer to Transmission Fluid Pan and Filter Replacement .

The oil is aerated.

Make sure the oil level is correct. Refer to Transmission Fluid Check .

The external oil filter is damaged.

Replace the external oil filter. Refer to External Transmission Oil Filter Replacement .