Bad compression in #5

Ran a compression test with my HF tester. I'm showing low low compression in cylinder #5. Came in at 35...Put a little bit of oil in there to see if a temporary seal would help... didn't do anything.

So how do I know if it's a head gasket or valve or something else? Doing some researching right now, but if anyone has some quick advice or links that'd be swell.

Thanks

You could do a cylinder leakdown test to see if you can detect where the air is leaking. Or at least hook a compressor up to the spark plug adapter for the compression tester for the same results. Then you could listen in the intake, look for bubbles in the coolant, etc.