There are a number of issues to check for on this:
1.) Valve cover gaskets are loose or the gaskets are gone, if these is on the valve cover with the breather, the breather may have issues (pressure build up in crankcase)
2.) Rings are shot and are allowing pressure to build up in the crankcase, (do a compression check to check)
3.) valve issues (either valve guides, seals or seats). This can be checked with a leak down test.
Another thought: when was the last decarbon job done on the head/valves?
That far up, there is very little that can cause it. The exhaust manifolds are at the very top of the engine, so unless oil is spraying pretty good from somewhere, which would be pretty easy to spot, its not from anywhere lower.
Check the breather gasket. It could be blown from the backfire, or it could have blown the hose off. Its right under the manifold.
Decarboning is generally done by removing the heads and blasting with walnut media.
Some guys have used Seafoam. GM makes, or used to make, something called "Top Engine Cleaner" that works well for this too.
Mercury makes Quickleen that should prevent carbon from building up in the first place, and clean out minor deposits. It works WAY better than Seafoam when used in the fuel for cleaning, and is considerably cheaper per gallon of fuel too.
Well, took top tin off. Can't see any obvious heavy leakage.
Couple of things....
Is it possible it's unburned fuel blowing out the leak in the muffler/exhaust pipe seam?
The plug on that side was pretty fouled.
Compression was 125 on that side and 100 on the other. Is that bad?
Spark looked weaker on that side also. Is the crack in the top of the coil a concern?
The coil should not have any cracks (age/heat damage), it lets moisture into the coils of the transformer. That crack is on the primary side but could be causing issues.
I believe you have the old style coil. Measured from plus to minus, disconnected from wiring, at 70 degrees (room temp), not a cold garage. The secondary is from plug tower to plug tower, look for corrosion down in the plug towers also.
Primary side resistance should be 3.7-4.60 ohms, the secondary side (plugs side) should be between 34.0 K (thousand) and 41.6 K (thousand) ohms.
Newer style style coil. Measured from plus to minus, disconnected from wiring at 70 degrees
Primary side resistance should be 3.0-3.5 ohms, nominal is 3.2 ohms. The secondary side (plugs side) should be between 18.5 K (thousand) and 22.5 K (thousand) ohms, nominal is 20.6K ohms.
If you replace the coil, the replacement needs to be moved to the firewall because of heat, and the tin needs to be capped to maintain air flow.
Now for the compression issue. You have problem on that low cylinder in relation to the other. Compression is minimally 75 -115 PSI maximum with no more than 10 PSI difference between the cylinders.
You will also need to change the oil and oil filter.