If your running a regular 40mm exhaust port, use this gasket shown below with the instructions I will also mention down below...I never had any exhaust leaks whatsoever using it and beats the stock gaskets by a mile...lol...It's only $5.54 at Amazon.
"Get rid of the stock exhaust gasket that came with the engine kit and clean off any other "goop" that you may have tried along with it.
Use this Toyota Highlander O2 sensor gasket instead, available at Toyota dealership or also at autoparts stores such as Auto Zone, Advance Auto Parts, etc.
Here is the current Toyota dealer parts#/stock#, for that gasket...89466-20020-83.
If you shop Autozone, Amazon, etc., its a "Dorman" brand part number ...Dorman 47021...
When installing this gasket, you will need to slightly elongate the holes that go over the studs/bolts...I use a slightly largerbolt to do this through those holes.
You will notice that this gasket has some metal affixed to both sides of the gasket...The side that has the most metal is what goes against the exhaust pipe itself...DO NOT use any gasket sealers with this gasket."