Subaru issued a recall (WWG-90) because the mass air flow sensor has a manufacturing defect.
Subaru Legacy Problem Reports
RepairPal Verified Subaru Legacy Problem Reports
The leak may develop from the oil pump seal. Our technicians prefer to use thread locker when replacing the seal to prevent the bolts from becoming loose again.
Front engine oil leaks can develop from the camshaft seal and the crankshaft front seal. If one or more seals require replacement, it may be wise to consider replacing the timing belt as well. These seals are commonly inspected and replaced as needed as part of a timing belt service.
The front oxygen (air fuel ratio) sensor element may develop a crack, causing the Check Engine Light to illuminate. Subaru has recalled certain models to replace sensors that may fail. The Subaru service program number is WXW-80, this is not a government mandated recall.
A vibration or shutter may be noted on acceleration. If the transmission torque converter is suspected a revised part is available to correct this concern.
A failure in the anti-lock brake system (ABS) causes the ABS pump to continuously run, which will eventually wear down the battery. An improved ABS relay is available to solve the problem.