removed my oil cooler housing (actually there's no oil cooler, it's a turbo, blah blah blah, but there's a housing) and it was sort of filled with silver pasty sludge! what is this stuff?? it reminds me of anti seize compound, it was all in the cooler housing, but i haven't seen a trace of anything like it anywhere else in the cooling system, hoses, reservoir, anything.....

 

Sounds like stop-leak to me.

 

I had it all over the inside of the housing on my 86. I never found out what it was, and it was a bear to clean off.
Stop leak will usually come out looking like kitty litter, and plug everything right up while it's in there.

 

Sound like the PO decided to half *** fix a leaky radiator. I would drain all the coolant and flush it out really really good.. That stuff can't be good. Then refill with new and it will be all better.