Of course, the intake gasket is not going to be tested with a leak down test, which tests the rings, valve seat sealing and head gasket. And the plug hole gasket of the test device.
It does not test the valve guide seals, intake gasket, exhaust gasket, etc.
#6 sure is ugly. i suspect the intake gasket. But, you have got more than one thing going on, it seems. Looks like
oil from here. Excess gas very rarely prefers to destroy just one plug. Ignition can have an effect, of course. But, that's kind of rare, also. That is, it looks oily, not gasy. You're there, you can tell.
You've got to learn how to adjust the valves, rather more precisely, if i might suggest. There is simply no way they could be so far off it they are done correctly. Get a book.
That #6 sure doesn't look like gas, but
oil. Did it pump blue smoke out the pipe? Did it attempt to clean-up in either a steady but aggressive right or left turn? (Be careful here. If it clears in the turn and the torque pops up, you are going to light the tires and then... understand?) There is no simple way to test the intake gasket seal i can think of at the moment.
i don't want to get all esoteric here, but have you measured the variation in the fit from the top of the intake manifold surface distance to the head surface, vs. the bottom of the intake manifold's surface distance to the head. That is, is there a different angle between the head and the manifold from the top of the manifold surface to the bottom? It is possible you have a wedge and a more open space on the bottom and less gasket crush, which will easily suck
oil from the galley. Particularly on a corner when galley oil sloshes to that side...
The intake gasket is pretty good and easy to install correctly. i don't think you screwed that up. i just am suspicious of the potential wedge angle between the intake manifold surface and the head surface.
By the way, they might be different on the left than on the right side. It could be the manifold surface. It could be the head surface. It could be both.
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i wouldn't even consider any sealant goop in the water system...no way. Get it correct. Some might disagree. Good for them.
At the race track, and you simply have to make the run, that is another story. But not on a new engine on the street, unless you are going to dump it and you don't mind screwing somebody. i have used it twice, only on old ratty cars that i don't want to really spend the money to fix and i am going to run to ground. Not on anything nice, except for a money race. Not even for sport trophys. i would rather sit the trophy-only event out than use that stuff.