Possible Causes:

WARNING: This page does not describe the selected car, but rather 9 other vehicles, including the 2005 Ford RV Cutaway, 2005 Ford Pickup, 2005 Ford F550 Super Duty, 2005 Ford F450 Super Duty, and 2005 Ford Excursion. However, it is still accessible from the selected car via links, so may be relevant.

Extended oil drain intervals - the anti-foam additives in the oil may be depleted from severe use or extended intervals.
Air may be present due to recent engine repair on injection control pressure system. It may be necessary to operate the vehicle aggressively for 24-32 kilometers (15-20 miles) to remove excess air.
Incorrect type or grade of oil.