I have oil leaking from my gasket valve on my engine. My car is 2010 Subaru Forester with 182000 miles. What sealant should I use?
Replace the valve cover gasket.
Don't use any "sealant". There is no product that will fix a leaky valve cover gasket. Replacement is the only way to repair it.
Make sure it's actually coming from the valve cover gasket and is not an external head gasket leak. That model is prone to blowing head gaskets and it usually starts as an external drip before it progresses to internal failure.
So to be honest. Replacing the valve cover gasket is beyond my skill set. Is there a plan B you can suggest?
Unfortunately the only way to stop the leak is to replace the bad gasket(s). Anything you put in the oil that would have any chance of plugging a valve cover gasket leak would destroy the engine.
Subaru's boxer engine design makes replacing valve cover gaskets more difficult than on an inline engine. Probably the thing to do is get a quote from a good independent mechanic in your area. Dealerships and chain shops are more likely to rip you off.