Is there an off-the-FLAPS-shelf recommended product, or just lots of water?

I am finally getting around to long overdue maintenance on my backup van ('88 Westy
AT)
after having owned it 2.5 years, and what came out was pretty dark, with some sediment
in the bottom of the buckets used to catch it. Also a thin slimy coating inside the
expansion tank, but nothing such as smell or oily sheen that might indicate internal
problems.
I will be replacing the expansion tank, cap, sensors, T-stat and some small hoses.
Thanks,
Jerry