the cooling fan test is pretty simple, you rev the engine, it will definitely let you know its working. It will blow an awful lot of air and will be loud doing it. (garbage truck sound)

you can refill it - but there is no scientific way to tell you how much. And the only test is to either install it on the engine or make up a test rig and spin it up. Too much trial and testing.

I have narrowed it down to 5cc at max as a good starting point and use 10,000 cst oil from an RC hobby store - aka 7000 wt.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)