Certainly worth a try!

 

It can’t hurt, as for the oil in the coolant if the engine oil isn’t increasing then it has to be the radiator has a leak in the transmission cooler. I’ve seen videos of the radiator being replaced and the engine cooling system flushed, the transmission fluid flushed , but I can’t say how long the transmission lasted after the contamination

 

I never even thought of using Marvel Mystery Oil or Seafoam. I'll try doing a Seafoam treatment and see how that does. Appreciate the help

I checked to see if I could hear anything from the adjacent cylinders but all I heard was it going down into the bottom of the block. It didn't sound or feel like the air was coming through the adjacent cylinder.
I guess if it turns out the rings are just worn completely, I guess it's time to learn how to change rings and pistons.

Looks like I'll check the transmission fluid to see if it's brown too.
I replaced the water pump with the timing kit, it was pretty difficult to get installed. I'll check that out and see if anything is going wrong there.
For cylinder 4, I'm guessing the oil in the combustion chamber caused the valve seat to get damaged?

For the timing issue - it only moved around a quarter or less of a stroke. Any way to check if the tensioner and other timing components are still fine?

 

The water pump might be the source of the coolant in the oil, as Donbrew said awhile back, if the gasket between the pump housing and the block was pinched or damaged when installed, or if the water pump is leaking at the impeller shaft seal.

 

I seem to remember there was a case of oil getting into the water through the timing side of the water pump, somewhere in the Wayback.

 

I’m 👀 as far back as I can

 

Drained the radiator and only got 2-3 quarts of coolant out of it. Then some oil started coming out. The coolant must be mixed with the oil too. What's weird is that I just changed the thermostat less than a year ago and the coolant was completely clean. I'm wondering how the engine stayed at normal operating temp with that little amount of coolant. Maybe the sensor couldn't read properly?

 

I drained the engine oil, and it all looked black and normal. I saw some of what I thought were a few metal flakes, maybe a dozen small particles towards the end. I don't know if it's really anything but I guess it's worth noting.
I also dropped the transmission pan and looked at the oil there. It is dark red, and there was no coolant to be found. Didn't make that much of a mess somehow. I guess the transmission filter, seal, and fluid change I was planning on doing last year can get done now, lol.
Haven't popped the thermostat off yet to let the rest of the coolant out though.

I found a borescope I have and looked into cylinders 2 and 3 and they're definitely burning oil. I gotta rotate the engine to get to the other cylinders but I'm guessing they're just as bad:

 

I forgot to mention that it looks like the issue I had from last year is still happening - it looks like oil is still getting sucked into the intake through the PCV hose.
One weird thing was that the hose for the coolant reservoir was covered with oil on the outside - not sure what's going on there. The coolant reservoir was filled with milky coolant oil too, having trouble cleaning that out.

For the timing issue with the leak-down test - is it normal for the cylinders to move from a leak-down test? I never saw any warnings about that. Is rotating the crankshaft a few times by hand a safe way to determine if it's alright to start up?
I'm stumped on where the oil could be coming from. How can I even determine if the oil is coming from a faulty head gasket or the water pump now?

 

The valves are getting close to "burned", leaking anyway.
Don't stress about the pistons moving; what do you expect when you pressurize a cylinder while the others are open?