Turbo 616 powered Astro Van build
Turbo 616 powered Astro Van build
It looks like rust/silt to me but remove a few freeze plugs and check out the old engine first. Ditto if it was oil it would be more frothy! The silt and rust settle to the bottom until they are mixed up by the water pump and moved around so the coolant will look clean. The rusty silt and anti freeze turn in to a mud paste like grease. oil alone won't get that thick. It will stay oily and float on top of the bottle not sink to the bottom like in your pic. also if it was oil it would be black. Take a paper towel a and rub some on and let it dry I would bet money it drys rust red