are you positive it is engine oil? Most likely rear main seal. you will have to drop the transmission to get into there if i am not mistaken. while fixing the seal, check to see if the oil contamination damaged any other rubber parts there and check the transmission for leaks to

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2001 E55

2001 E320 Sedan

2001 Subaru Legacy

2006 SLK280

2016 Subaru Outback 3.6