I figured I'd post this up in case anyone ends up dealing with it. I've been lurking on the forums and signed up the other day. I found many threads where people talked about bad chain housing being eaten through from failed tensioners or bad guides.
Well, I just got this toyota 4runner on the ultra cheap because a previous owner didn't want to do the chain, and his wife hated it.
I saw there was indeed a little milkshake inside the radiator but the oil looked perfectly fine. I read tons of threads. When I took my cover off, I dismissed it as "close, but not the case." But.... I went back out tonight, put my glasses on and really gave it a close inspection. What I saw were some very fine cracks that if you didn't have at just the right angle in the light, would probably miss entirely.

When I flipped it back over, there was one spot where the chain was slapping more than other areas, and a clear area where the aluminum had been pushed in, which I had earlier dismissed as, "Maybe that's just how they're made" since I'm unfamiliar with these. But sure enough, there is indeed a fine crack that has formed in a dark spot where it got hot, pushed, cracked and nearly wore through. I am shocked that this engine ran as smooth as it has. I actually drove it home this way, against my better judgement, in a bit of a "Yolooo send it!" moment. The chain had a LOT of slack in it when I pulled it.

Already replaced it, already have a new housing, new gaskets, new pumps, so I'm good there. But I did want to share my finding with others in the event someone is having oil in the coolant, and a broken guide or tensioner, and is going, "I mean it LOOKS fine but I can't explain it." I wasn't able to get my phone to focus incredibly well but if you look, you'll note there is a chain shaped "push" into the housing, along with very fine cracks that have formed there. Like I said, on first glance, I dismissed it, it didn't appear to have "worn through" and while it isn't all the way, it's small enough to let the small amounts of oil in that I've been seeing. So if you DO have failed guides, and oil in coolant, double check those housing areas, it may just be small and you don't see it.

Crack that formed from chain wear on housing

 

Good info thank you