Good afternoon all, Happy New Year, and we have a tentative explanation for what was leaking into the cab. And it's a relatively easy thing to fix.
First, it wasn't the big baddie possibility: a crack or leak somewhere which was allowing the oil and coolant to mingle. Whew! The oil looked normal, the coolant looked normal, the truck fired up just fine, ran smoothly, and the leak didn't accelerate at all while the truck was running. In fact we figured out that the leak is very, very slow - maybe a drip or two a day. When I checked the oil, I suddenly remembered that we'd changed the oil right before the covid lockdown in March, and I haven't driven it even 20 miles since then. The oil on the dipstick looked absolutely fresh, that golden hue as it comes out of the bottle. No milky appearance at all. The coolant also looked a normal shade of watery lime green, no dark discoloration in that either. Compare that to what we are (still) getting in the cab - thick, black, stinky stuff.
Then my husband pointed out that the cab itself seemed damp. We do live in the PNW, it's the middle of the wet season, we haven't driven the truck in months and it lives outside. All that is a recipe for condensation. And then finally, the last piece of the puzzle became obvious when I saw a dark little furry shape scurry across the floormats as I was checking other things. I sniffed the leaking gunk, and much to my disgust, picked up a whiff of .............. rodent urine.
So what we think is happening, is the truck has been sitting closed up long enough, that rodents moved in at some point. And the truck is sitting on a slight incline towards the driver's side, such that rain will occasionally seep into the passenger side door frame, and possibly around the windshield. That water sometimes ends up on the floorboards but I wouldn't be surprised if some gets behind the dash too. Combine that with the occasional rodent taking a leak wherever he/she happens to be standing, and 38 years' worth of dust and grime and dirt building up behind the dashboard cover, and voila. Thick wet black liquid that pools against any given barrier behind the dashboard cover, then drips along the edge. Yum.
So we're going to pull off the dashboard covers, clean as much as we can, put mothballs in the cab to drive out the rodents, and figure out how to store the truck under cover. Or maybe just buy a cover for it. And get on the ball to finally get our known leaks dealt with once and for all. But thankfully, the truck seems to be mechanically sound. Just watch out for those <expletive inserted here> rodents.