If it is the rear main seal(RMS), then it is caused by too much crankcase pressure. The crankcase pressure is caused by a clogged PVC system which may include the PTC nipple on turbo engines or just a clogged oil separator box.

Cleaning out the hoses and box "could" stop the RMS leak if it is not leaking too badly yet. A quarter size drip or less.

Replacement of the RMS is costly for a $35 part, the tranny has to be lowered which is not a fun job on a FWD car. Depending on labor charges, $750-1500.

The PCV system still must be done and can be a 5 hour DIY job, or a 3 hour mechanic job if the mechanic is a Volvo person.

There are reports of people using AtuoRx and letting the chemicals clean the engine and PCV with success. I believe it might work, but replacing the oil separator will work.

I "stole" a PDF that shows the replacement for the knock sensors, they are also on the front of the engine block and shows the work needed to replace the box and lines. Just slightly simpler for a NA engine. It is in your Email box.
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My name is Klaus and I am a V ♂ lv ♂ holic.